I didn't think this would end up being my first post on the BYOAC forums (since I'm in the process of designing my first cab) but I just got through playing this for about half an hour with my girlfriend, who doesn't really play video games much, and it was a blast so I thought I should post some encouraging words!
Keep the game simple. Keep it hectic and fast paced. Keep it kind of "cute". Those are strong points of this game! One of the comments that came out of my girlfriend's mouth was "It's like bumper cars!" and I thought that was definitely appropriate. We had it set on 2 players and 2 AI. We enjoyed tag-teaming the AIs and making them spin and spin and spin...
Here are some suggestions for further development, if you wish to hear them:
1) She found the black tank to be too hard to see against the background. I had to switch her controls to the yellow tank so she could play.
2) Have you considered making the walls semi-transparent, or perhaps translucent, so that a player can still keep orientation on his tank?
3) As a possible scoring option, rather than being an elimination where you have a limited number of lives, you can earn points for hits and play for a timed round. Also, one way to encourage different types of shots would be to give a score multiplier based on the angle of your gun. The higher your gun angle (and hence, the farther the shot and the harder to aim), the more points per hit you can get.
4) Another possibility that would be interesting is every few timed rounds, you could have a "race" round where the map is a track, and the first one through the track or maze gets bonus points... shooting scores and timer would still apply. Or you could have them keep going around and around until the regular timer runs out, and each time you pass the "finish line" you earn bonus points based on your ranking. This would mean that camping the finish line and sniping people would be a valid, and hilarious, tactic.
5) Different wall heights, so you would have to lob shots. Differing wall heights within a given map would be interesting too.
6) A "fortress" map where some players are inside "turrets" and some are in the "field". Imagine starting positions 1 and 2, with their wall bracket extended into an enclosing square, and then elevated above the playing field. Field players would have to lob shots and turret players could drop shots and would have the advantage of elevation, but would also be limited in their ability to escape shots.
7) I like the way the angle controls the power currently, but what about a "power-up" shot where if you hold your fire button for, say, a full two-count, you would shoot a shot (same speed and power as a regular shot) but it would have a blast radius, like a firebomb?

After "inserting coin" the player could choose from three different tank types (with slightly, though equally cute, body styles)... a "standard" which is the current tank, whose power-up shot is a firebomb; a "heavy" which moves slower but which tilts less with acceleration and spins less upon being hit, whose power-up shot is a "concussion" shot (greatly slows all tanks in the blast radius for a period of time); or a "scout" tank, which is faster but spins longer on getting hit, that has a "halon blast" shot that prevents tanks from firing until leaving the blast radius which lasts for a certain amount of time
There's just so many things you could do with this game! Pretty cool, I hope my ramblings have given some inspiration. I look forward to seeing this project continue... I will -definitely- be adding this to my cab when I build it... it has a ton of "party game" potential!