The worst things;
1) Those character archetypes... you know, the big-breasted shy girl, the flat-chested hyperactive/high energy girl, the loveable ditz, the smart one. It's like a friggin' boy-band. It really robs the characters (and the shows that use these) of any true character development.
2) Fan-service. Stuff like panty shots or still-frame "uahh" scenes that serve no other purpose other than to satisfy the demand of fan-boys who ask for them.
3) Motionless animation. You know, when NOTHING is moving except one persons mouth. Everyone stops blinking
4) Flashbacks. Or flashback episodes designed specifically to show you clips from previous episodes. Either the person watching the show will already know these things or a person who stumbles onto the episode won't care. It's a lose-lose thing to have.
5) Harem themes. The idea that 3~5 different women would fight over one loser. The new girl, the old-girl-next-door, the cousin/relative but-not-blood-related (or sometimes are), the 'I hate him but I need his attention'/'I secretly like him but I act like I do not), and the secret-crush/crush that cannot confess. It may have been interesting ONCE, but there are SO MANY animes that are JUST THIS. Character development is nonexistent. Probably the worst anime has to offer.
Also, #5 usually comes in the variety of #1. So it's twice as bad except it sometimes has #2 in it too. They all have #3.
Filler's pretty bad but some anime (the one-season only types) don't usually have them. It's only the story-driven ones (Naruto, Dragonball Z).