Non-fluffy armies.
By which I mean Armies that don't follow the background.
I know its a small thing, and maybe some people play warhammer to Win, but with all the background available and all the characters and all the stories and open ended hints in a codex, I would have thought people would collect armies that they feel a kindred with rather than the ones that look the most powerful because of some beardy rule.
Now I can be very tactical. I know about flanking, ambush, deep striking, scouting, sniper, tank hunting, using cover, decoy units, meat shields, double decoy. I ge thow they work. But i refuse to infiltrate my berzerkers, because I can't see it working fluff wise.
And mixing Khorne with Slannesh, or Tzeentch with Nurgle, or space wolves with dark angels (I think theres emnity there...) or necron/orcs/tau/tyranids/Eldar with anyone.
...I know its unfair, as some people do just want to play to win and so build the most competitive army, which is fine, but leads to many people leaidng the same armies and thats just dull. Get some maverick armies together, An Orc army consisting of nothing but gretchin, snotlings and one Big mek with a shokk attack gun. A space marine army that doesn't use terminators. A Chaos hoard of troops. A Kroot army, with a modified Shaper for HQ.
Gah, i'm getting ranty now...