A jumping tank… quite funny concept.
The tank is pretty static and balanced even when you fly all around the place, but does crazy stuff when you drive on an edge. It also goes forward/backwards really fast in comparison to the rotation speed, but since it's a platformer, I wouldn't change it. The reason you don't see many tanks in platformers is because they're quite long, which is a downside if you need to land exactly on your centre of gravity.
– Try to shoot a pile of your on bullets somewhere and go over them (good place to do that is the red container in level 2).
– If you jump from a diagonal surface (the ramp in level 1, any edge), you stay askew even mid-air. The tank probably ought to have some gyroscope (= in mid-air you return to your default position). It'll help you make more precise jumps (for example, in level 1, try to jump from the ramp back where you came from to see what I mean) and avoid ending up like this:
http://www.newgrounds.com/dump/item/3c9a2ca6bcc34cb75a485556283206bd
– In level 7, I thought that was a wind at first, but it's actually a sliding platform. It should carry you along if you stand on it.
Your bullets should probably explode on impact with everything, not bounce (and kill you if you are way too close). Also, you can shoot through stuff if you are close enough (try that in level 1, with the snow covered trees). Good idea might be to deduct a point if you miss a target, so you don't end up with people all having maximum score which is now quite easy to get.
Colliders.
– The tank. It's good idea to ignore the cannon, so you don't get stuck everywhere, but the bottom/front/top turret should have more precise collider borders.
Also if you jump into the vertical side of a block (try the start platform in level 5), you can "hover" in the air for like a second as long as you hold the "forward" key.
– Level 1, "bark trees" right at the start have no collider.
– Level 5, try going straight forward. You won't be hit by the spinning fans unless you jump.
– Level 6, try shooting in front of the first lava pool. It will bounce, plus the bullets will lie on the flowing lava and not sink down/move/explode:
http://www.newgrounds.com/dump/item/fff86a1dca88d61c763cc62781163984
The shooting sound might be a bit annoying, especially if you need to shoot many times in a row.
You're probably going to change the environment, but anyway…
– The "blue" portal lies on a "blue" blocks, thus its borders aren't visible (in level 2 I first considered it to be a shadow or a hole until I drove over it)
– I wouldn't use yellow blocks as platform in order to avoid possible confusion, since you already use yellow as "win" platform.
– The background is also quite blurry, as it's probably a low-res, scaled up picture, but more importantly, the background is a box with quite visible seams.
I hope you'll find some of these helpful. Good luck.