I was kind of hoping from the name and the promise of beating up lots of enemies that Thing-Thing would kind of be like Xiao Xiao (which, while technically not a game, is undeniably pretty awesome). Sadly, I was disappointed, as Thing-Thing 2 is a relatively straightforward and boring beat-'em-up.
So, there's you, and there's an infinite stream of enemies. You can walk, jump, or punch (pretty ineffectual), or shoot them with one of the wide variety of weapons in the game (point and click). The one thing that makes shooting enemies nontrival is that each weapon has its own recoil, which will pull your cursor away from where you want it to be if you're firing a bunch of shots in quick succession. (It is amazing to read all of the comments complaining about how terrible the aiming is. If you took away this feature, I can't even imagine how pointless the game would be.) Ammo can be annoyingly scarce. Since the enemies don't do very much damage even if you walk amidst their throngs, you might wonder why you need to bother killing them at all. The answer is that there are doors that require you to have killed a certain number of enemies before you can pass through and continue the level.
The game offers a couple of modes. There's the rather-misnamed Story Mode, which offers no story to speak of, but I guess Sequence of Seemingly Unrelated Levels Mode might be a little long to fit. (To be fair, you do gain weapons as you move from level to level, so it's not completely unconnected.) Survival Mode is pretty much what you'd expect.
The graphics are pretty basic -- all of the characters are just built out of spheres, although the game gets credit for making a lot of unique enemies. The sound is pretty much your average boring gun effects, while the music is OK -- it's at least somewhat less boring and repetitive than a lot of Flash games, but it's not great either. However, none of this changes the fact that the fundamental gameplay is simply pretty boring. I didn't really enjoy playing this game all the way through, so I'm glad it was short and I was able to earn the badge in a reasonable amount of time.