Gnome warrior tanks disturb me. In fact, I'd go so far as to say I just can't hang with a gnome in this role. Especially in PvP content, their size relative to other PC races sends any semblance of reality out the window. In raids, their small size makes them annoyingly more difficult to spot. But perhaps the most disturbing aspect of this role/race combo to me is how much less confidence-inspiring these tiny tidbits of tanking are to me than, say, an orc warrior. I'll follow my orc warrior anywhere, even the proverbial cliff, because I trust him to fight off the baddies once we've rezzed at the bottom. But a gnome? Sorry, guys and gals. You're like Chicklets chewing gum. It's just not working for me.
I have a friend who doesn't like to see human males in magic-using roles. He has a pretty classic view of spellcasters as refined in both intellect and physique, and he just doesn't enjoy seeing WoW's rather beefy-looking male models flinging spells in the field.
Whether your reasons are aesthetic or practical, have you found any particular race/class combinations to feel a bit dodgy? Is there a race/roll combination in game that just doesn't work for you?