The whole Gen IV:
- Evolve by location. If I were in Pokémon world, I'd pick a Fighting Pokémon and made it to smash the hell those moss/ice rocks in pieces, then pick them up and sell them at a high price.
DefuckDefog. Not so problematic in Route 10 when my Flying's moveset is not complete, but annoying when I'm going to Route 224.
- The lack of fire types.
- Amity Square. I share the pain of the guy at the lobby complaining for his banned Gyarados and Steelix. At least it was half-fixed in Platinum by allowing the starters to enter. (still ironic you can carry a giant 700 lb. turtle with a tree in its back and not a little, cute cat/fox)
Only Pt: I'm trying to think and... nothing at all. (except by aforementioned Defog and location evolution)
- No Vs Seeker. I hate you, Route 30 Rattatard.
- BP for tutor moves.
- Unobtainable Leafeon, Glaceon, etc.
- Lyra (or Ethan if you're female player). Most useless gender counterpart ever. Too obvious she/he was forced into the plot. Better they wouldn't include her/him at all (like FR/LG or B/W)