I have nothing against monks. They are eastern-flavored, but for me D&D has too many historical inconsistencies to be treated as "european" anyway. It's just a fantasy world. Ki-powered martial artists are no better and no worse than spellcasting clerics.
I have a mild dislike of assassin, but it's not because of the class itself. I just dislike how the classes are designed, with no clear indication of whether they indicate abilities, profession, archetype or style; we get a mix of all of these. If we are talking about profession, assasins are fine, but not appropriate for player characters. If it's a set of capabilities, it's just a type of rogue.