Almost perfect...
...but there are a few things that in my opinion significantly take away some of the fun of Survival mode:
1) Enemies can spawn right on top of you, dealing you inescapable damage; you should never be given damage simply by an enemy spawning. I'd suggest either making sure they're a reasonable distance away, or preventing them from attacking or dealing damage for a few seconds after spawning.
2) Enemy melee attacks always hit, even if you're a hundred pixels away when the attack animation finishes; you should be able to dodge their attacks by being quick enough.
3) Every so often I'll have a game where it seems like bows are incapable of dropping. I just played a Survival game while wearing Level 6 Armor where I got a long bow at about kill 40 and a crossbow at about kill 230, with nothing in between (I also ran out of light arrows around kill 100 and didn't see any more again, ever). By then I'd taken too much damage from enemies being practically invulnerable for me to stand any chance of reaching 5000 points; I count myself lucky that I reached 2500. I think it should be hardcoded in that when playing Survival mode, bows drop at certain times (i.e. long bow at kill 25, composite bow at kill 70, crossbow at kill 150, compound bow at kill 280) to make sure the player doesn't end up in an impossible game by pure chance.
4) Need hotkeys for switching between arrows, especially in Survival mode. Half the time when I moved my mouse down to click on an arrow I went too far and clicked out of the game window, which totally screwed me in the game, plus you have to look down at the arrows instead of at the things that are coming to kill you in order to do it. Alternatively, making it so switching to a bow automatically switches to the most advanced arrow that bow can handle would work fine too.
5) A save/load feature would have been nice. For those times when you start a game but are interrupted and need to turn off the computer, so you don't lose your progress.
Also, a weird oddity: during my Campaign game in Level 6 Armor, at the beginning of levels 15 and 25 my health just dropped itself by about a third with no reason that I could see. Weird.
Also there's no sound after clicking Retry. FWIW I'm using Firefox 3 with Flash Player 10.
Those things aside, this is a fantastic game that kept me interested for what it tells me is 7 hours and 49 minutes of playtime. It reminds me of Diablo 2, since so much of the game is based on moving in the correct way to herd the enemies where you need them. Having just failed in Survival mode about 10 times in a row, I'm now going to take a break and come back for my final medal in the game at another time.