Newgrounds Rumble is a fairly straightforward brawler in the classic tradition. However, it doesn't seem to be particularly demanding of its players; while there probably are layers and layers of strategy, I was able to get through the game mostly just by mashing buttons (and some persistence in the more difficult challenges). I don't even know if the characters have special moves; though there are two only kinds of attack ("quick" and "fierce"), your characters do have different moves while in the air and you can put together some pretty impressive chains.
Overall, the game is well-crafted; there's a fair number of characters each with their own distinct personalities, and there's a wide variety of arenas. The game handles up to 4 players, and some of the challenge modes include both 2v2 and 1v3 play, which is nice. There's also a lot of game modes -- the obligatory "story" mode (which is pretty skeletal, both in story and length), challenge mode, ordinary versus mode, and an unlockable survival mode. The powerups available are pretty standard for a Super Smash Brothers-like setup.
There's a fair amount of different music in the game, and while none of it is great, it's all decent. The sound effects are pretty limited but not bad. But really, the most baffling thing about this game is simple -- why is it on Kongregate? The game is clearly packed full of Newgrounds in-jokes and references, and as someone who hasn't spent too much time on Newgrounds, most of these flew completely over my head. I'm sure it would be more entertaining for someone who knows all the backstory behind all of these characters and the various references in the game, but to me, it ends up as just another fighting game. Not a bad fighting game, mind you, but just not one which is interesting enough to keep me playing.