Hmm... Besides super effective critical hits, magikarp, zubat, swoobat, finding feebas, and having to answer everything with either a yes or no?
Boy in hoenn: where are you from? Y/n
-picks yes-
Boy from hoenn: yes town? Never heard of it.
I think it would either be Whitney's milktank, having to trade in order to get some Pokemon to evolve, freakin ludicolo that spams double team, and the inability to forget HM moves without a move deleter guy.