To me, the greatest joy is PvP. Going to Naxx is one thing, grinding is another, but finding an alliance dood whacking some mob in the distance, jumping on'em and blasting his *** off while he's like "WTF!?" is THE No.1 enjoyment this game can give me. It's priceless to feel the shame you put on the enemy when he's a pally leveled 3-4 above you and trying to gank you while grinding and you STILL kick the asses of the mobs and the guy.
That's what I like about this game. Mobs are just mobs, stupid AI controlled piles of 0's and 1's that will respawn in minutes and give shlt about you killin' them again and again.
Wasting the day of an enemy player and turning his char into a door-mat, THAT's something enjoyable.
And for all the above mentioned situations - Frost is teh only way to go.
Sure, I could crank up twice more damage with Fire, but Fire won't give me 21 sec FN cooldown, improved chill effects, water elemental with all his abilities, Ice Barrier or Ice Block.
Against players in world PvP, fire is a no-go*.
I don't care if the mob thinks me good or bad when i'm killing it the 1000th time.
I'd rather have the enemy player think "shlt dat dood is 3 lvls under me and still prawned me wtf nerf mages /cry !"
----------------------------------------
* - I've successfully grinded and PvPed with fire all the way down to lvl 51, but PvPing with Fire is just lame compared to Frost - you run around a lot instead of shooting cause you have far fewer ways to stop the target. And when you get jumped on... you're mostly screwed without Ice Block and Ice Barrier. And please get lost with Mana Shield, that thing is a choice between "I die now with a lot of mana" and "I die in 5 secs because I'm out of mana". It soaks up less than Ice Barrier, sucks hundreds of mana points in the process, lasts only 30 secs and has 30 sec CD (you can't recast it when you need to and can't have MS both active on you and ready to be recasted, unlike Ice Barrier with its duration of 60sec and Ice Floes'd cooldown of 24 sec).