This is from the casual perspective, seeing as how I don't attend tournies on a regular enough basis to form an opinion on who I wouldn't want to play.
The one person I've really had a problem playing is this guy by the name of Chris Scroggins and it was all due to the fact that he always made the stupidest plays possible, and he'd always get his grandma to buy him some really good old school stuff, not P9 but a few steps below that, and it was just really annoying, especially when you take into account that he had been playing for about 4-5 years.