Kickstarter Bonuses - What Do You Like to See?

Beta/Alpha access is probably the biggest thing for me. I also think plushies get a lot of support :b

for me it would really be to that the dev will actually finish the game

^this, that the game launches on any platform finished, and not early access or ultra mega alpha 0.003 xD

All of the above. Betas, platform tests, special features for backers, yeah.

A lot of people like to just pre-order the game using kickstarter, so if you want to tempt people to spend a little more, space your extras tiers close to the pre-order tier, for things like soundtrack, beta access, etc.

One thing I've seen done wrong quite a few times is assuming that everyone values beta access more than artbooks or soundtracks. It's not necessarily the case. For instance, I only really dig the soundtracks and unique skins, and on a rare occasion the artbooks. One way to get around this might be to make a basic pre-order tier, but then offer "Add-ons" for soundtracks, artbooks, beta and alpha access, unique skills/skins/characters/whatever, and then let the backers pick from what they want. That way you can get that little extra pledge money, and your backers can get what they want without being held back behind the cost/gain barrier. If you plan on doing cross-platform, another good add-on is "extra copy, for different platform". Be careful about making add-ons for extra copies though, because if you're not careful people will pledge $1 in base, just to get the add-on price, sometimes even undercutting your "Early Bird" tier.

Another huge mistake is setting the kickstarter pre-order price too high, and then by the time the game makes it onto something like steam's Early Access, the discount there can be better than what your backers in KS got. It's not so much a problem for people who love getting in on alphas and betas, but for people like me who prefer to abstain so we can experience the whole thing without getting burned out on the project we love? It ends up being a liiiiittle bit raw deal. At least, if you don't have much disposable income for entertainment.

Kickstarter unique content is a pretty good way to pull interest. It's always better if it doesn't create a pre-launch form of "pay-to-win", but rather gives your early backers a little extra customisation. These such rewards need to be exclusive though, otherwise people start to feel cheated. You can avoid that by clearly stating that the extra content will be available as DLC later/at launch, and by offering it at a solid discount for backers. For instance, Contagion gave backers some cool ringtones for use in-game (for a sort of close-range marco-pollo mechanic using bluetooth). The trouble here is that non-backers had to navigate around these locked-out selections to pick their own, and it felt a bit weird. If you can find a sweet-spot for unique and interesting content and what level of exclusivity it has, you'll do well! One campaign assured us that certain crowdfunding extras will be totally exclusive and not released as DLC (though they did plan for other DLC later), and as a result I was far more tempted to up my pledge. I wouldn't mind so much but I was taking a risk to begin with because it's not the genre of game I usually enjoy, and they totally manage to get me in on it. (It was Armello, and the four exclusive bandit heroes)

I can't really comment much on higher tiers of things like figurines or letting backers into the design process for an item/NPC of their own, but "Early Bird" tiers tend to do well. Some kickstarters even had success making tiers lower than the pre-order, for just the soundtrack. Some people aren't big gamers, but can appreciate the musical talent (assuming you already have someone working on that so you have samples for the pitch video/page).

I would suggest googling around to find out about successful kickstarters as well as failed/scam-like ones. You can then go visit their original pages and check out their front-page layout, their pitch video quality, and what kinds of things they put into their updates. Even if you're genuine, some people may have concerns, because gamers do get burned fairly often by crowdfunding/early access and broken promises. You may even decide it's better to hold off on the crowdfunding until later on when you have more content to show. Example content is a big deal in getting people's attention. Anybody can sell you an idea, but it means a lot more when you can prove you have the focus to turn your idea into something tangible and with the correct feel and quality.

Pretty much what everyone else has said (especially skins for me - they're something extra but don't break the game for people who back at lower tiers). Depending on the game, perhaps a free Android/iOS version for the higher tiers.

soundtrack (.mp3 and other formats), digital+physical copies, modding support (Built in feature for ALL people, not something like "n tier gets modding tools". Actually seen that on one game. Mulyplayer maps,devkit maybe?

guides/artbooks/etc stuff like that.

Doublepack for early backers.
Beta access
Physical goodies

Exclusive in-game vanity items, like what Sapience is offering for Extra Life.

I remember a company who made a puzzle game that I was interested in. I was about to take the first tier with steam key, which can be thought of as a pre-order. Then I realized that there were bonus levels made for kickstarters, but only for people who took a tier 50% more expensive than the retail price of the finished game. In my case, this bonus worked as the opposite: I don't like the idea of backing a game and not having all the gaming content (I'm speaking of playing time / levels, not of skins or other cosmectic item, nor art, or music, or everything else). So I didn't back the game (I finally bought it in a bundle for less than half the retail price, and only 1 or 2 months after it was available, but that's another story !).
So my advice would be that: do not include playable content that is tied to a specific tier. Of course, adding some playable content to all tiers, as an exclusive backers bonus, can be really good (not going into details, since Uroboros already did it very well).

Promise to get all games from this developer for next 10 years for free (i saw this bonus in Pathologic kickstarter). But this works only if you already have pretty strong reputation.

What I ALWAYS like when browsing RPG games on Kickstarter is the bonus of adding the backers at a certain tier into the game as NPCs and put on your own scripts or quests.
But of course, I had never backed at such amount.

Other than that, a t-shirt or any clothing/accessories is good enough.

There are 2 things i wish to see when i donate in Kickstarter:

1) I would be really pleased if the developer was sending me a newsletter once a week while the game would be in alpha and beta and once a month when the game will be released.

2) I would like the developer to actually and honestly remembers me. This can be done by sending a "How are you?" no-reply email every 3 or 6 months or by sending me an email the day of my birthday for at least a couple of years.

I donated 2 times on Kickstarter, both times i gave 5$ on 2 separate games. The only thing i received from the dumb "developers" (i don't wish to say a worse adj about 'em) were a few badly made newsletters about the beta release and the final release. No beta access, not even an appropriate "Thank you so much!" reply. The result? I did not buy any of their games and i am not intended to do so!

So my main advice: Respect the people who will support you and do not forget them

For me, I'm very unlikely to pay more than the minimum. I'm in fact very unlikely to back a kickstarter project to begin with, unless it's so cheap that I feel that I'm willing to risk my money on a project that may or may not be released. How cheap depends on the track record of the developers, if it's someone who I've seen do well in the past, I'm willing to pay more than if it's someone who I know nothing about. Paper Sorcerer is an excellent example of an "unknown" offering a game that looked interesting enough, at a low enough price, for me to be willing to part with some money (and I don't regret doing so, Paper Sorcerer might not have been perfect, but it was fun and it was interesting) .

So for me, unless your higher tier offer is something absolutely amazing, I won't part with any extra money. And that "absolutely amazing" is unlikely to be a realistic bonus.
Actually, before submitting, I just came up with one thing. Soundtrack. If you give a soundtrack sample, and it sounds like something that I could use (likely as background music for my pen & paper RPG sessions), I might part with up to €2.5 extra. But not more.

Art books in pdf format, instruction booklet/guide also in pdf, sound track, reduced price from final release for being a backer, beta access, and alternate player skins that don't alter gameplay but are exclusive to backers are my favorite rewards.

I'm not particularly fond of the Kickstarter concept in general and I really think money shouldn't be wasted on this kind of things.

Focus on the game, make it good, ship it. That's my 2 cents.

I back games on Kickstarter more than I buy them on Steam. And I tend to pledge more money in the average (and even the median) Kickstarter pledge than in the average purchase either on Steam or any of the bundle sites.

However, very few games get me to shell out more than the bare minimum needed to get the game, once it is done.
One part of it is the fact I'm not a rich man. Not by a long shot.
The other part is that I'm not very much interested in the frills, at least not up front. Not in the majority of cases.

For Project Eternity - now Pillars of Eternity - I shelled out $35. It was the first time I pledged, and was too late for the early bird tier anyway. An extra $10 for the soundtrack and monster manual seemed like a good deal. Later on I added another $20 for the expansion; that option was introduced a bit later and seemed like a good idea as well.

Torment: Tides of Numenéra I backed early enough to be in the early bird tier; IIRC I was among the first thousand backers. However, that was Torment. I pledged $80, partially because I'd missed the Wasteland 2 campaign, but mainly because it's freakin' Torment. I'd have given more if I could, and I've given a bit more in the past days (and I also got a few friends to pledge at least $20).

All in all, while additional content generally goes down well, don't focus on that. Focus on the game you wish to make; sell the concept, and people will give you money to see it happen. (Also, deliver. That should go without saying, but you also have to sell the notion that you can deliver.)
Whatever you do, however, be very careful about adding special in-game items for high pledge levels. It's generally considered a dick move and people often shy away from pledging altogether.

Also, some people like physical rewards. I don't know how feasible that is for you and your game concept, but some people like merchandise and can afford it. Depending on your target, that may be a viable option to explore - but only if your goal is high enough.