This would be an awesome game...
...but the different weapon speeds mean it's not a game of skill. It's a game of "pray it doesn't keep sending 2 projectiles at the exact same time, with you being forced to block one and soak the other." If this happens enough times on the same level, you're dead, period, no matter what you do to try to stop it. Now take a 50 level game with 90 second levels, and the only way to win is to go nearly an hour and a half without this happening to you a single time. Add in the fact that there's no form of save option, and you can virtually guarantee that no one will beat this game.
It's too bad too - the different tanks were well thought out, and you'll actually find yourself thinking about what order to kill the tanks in - "Do I keep that flame tank alive so I have more firepower to work with, or take it down ASAP so I'm in less danger?" The special upgrades again give tactical options - particularly the choice of whether to go right for some protection, more explody power, or investment so you can save up more money for basic stuff later.
The shield seems majorly overpriced - and the reduced repair bill hardly makes up for it. I did notice the regen rate improves as well as you level it though, so I'm guessing that's another 'this is weaker initially, but stay with it and it's the better option." Again, this gives some nice options.
The catch and shoot mechanic was very well thought out, especially with the speed increasers - someone going for more speed and less damage will find missiles a better catch choice than fireballs, etc.
It's clear a lot of thought went into the gameplay on this one, and this WOULD be one of the best defense games ever made.
Ultimately though, you're a slave to the RNG, and it's only a matter of time before you get enough shots hitting at the same instant that you're dead, game over, no save point, there goes an hour. A few if statements in the code to make sure it can't fire anything you can't block, and you'd have an awesome game.