Hmm... It's kind of a reverse for me, I suppose. With certain armies, there's specific ways I feel I "have" to build them. I.e. Nurgle armies should be in multiples of 7, and they shouldn't have infantry equipped with heavy weapons. With Iron Warriors I don't take Daemon units (except Oblits, which are kind of an oddity), and I don't take assault units.
I also tend to avoid the "must-have" units like Heldrake with CSM, or Lootas for Orks. Granted, with the way the Orks were redone, Lootas are too useful to avoid. But I just dislike people saying "You must have this in your army to win."
Centurions get a block from me on the price. Seriously, that's insane. There's a few other units like that, too. Mek Gunz aren't included because I can convert my own; similarly, I already have a bunch of MegaNobz. Flash Gitz are waffling on that wall.