6. Rhythm Games
Flash rhythm games are the poor man's Guitar Hero. Literally. There's no reason to play them if you can afford Guitar Hero. Rhythm games are never more fun when you're not holding a fake, plastic guitar. Flash rhythm games have too much working against them. You need head phones so you can't play them in class. Pushing A, S, D and F to a melody doesn't resemble any real musical skill besides tapping your fingers on the desk. Flash developers can't afford to license real songs. It's unfortunate, but the saddest part is that flash developers keep making them. They waste the most time of of all. Not all rhythm games are bad, there are some great artsy and genre-combining examples. But the Internet would go on just fine without all the lazy Guitar Hero clones.
5. "Escape the Room" Puzzles
If flash games are any indicator, flash programmers are severely claustrophobic. Maybe they spend so much time in one room that they like to pretend they're not there by their own free will. Whatever the reason, there are hundreds of games that start with the premise: you awake in a room, you're not sure how you got there, the door is locked. Escape-the-room games are a happy marriage of point-and-click adventure games and noir fiction. Click everything in the room then figure out how things fit into other things until you get out. The only characters are you and your inner monologue. Sure, there's a level of Resident Evil "Why is this crank that unlocks this box here?" but if these games made sense, you wouldn't be locked in a mysterious room in the first place.
4. Upgrade Games
Upgrade games aren't so much games, as they are menial tasks. You don't beat an upgrade game. You buy the last upgrade, see what it does and find no reason to continue. They're usually some kind of shooter, be it space or overhead or zombie, require a minimal amount of skill, and suck out an ultimately unfulfilling hour of your life. These are the lowest form of flash games. Your powers increase at the same rate as the difficulty, so the game never really changes. It's like someone thought the grinding from WoW or Diablo would be better without variety or camaraderie. Next time you find yourself saying "I'll stop after I see what the laser bazooka does," x-out the window and look at porn instead. At least you'll get a legitimate reward when you finish.