I leveled to about the mid 50s as fire because I didn't want to give up the incredible damage I did with combustion, ignite, pyroblast, etc. It is hard to give it up. If my pyroblast got a crit on a same level monster, the damage was insane and all it would take was a fireblast or maybe a fireball to finish. However, people kept saying, "frost frost frost..." and there were places like Searing Gorge/Burning Steppes I wanted to visit that had plenty of fire resistant minions so I made the change reluctantly.
I didn't like the damage drop at first since a frostbolt can't come close to a fully talented fireball in terms of damage(especially with ignite crits). However, having ice barrier, Water Elemental, frostbite, shatter, ice shards, cold snap, you name it, it didn't take long to find that leveling became an absolute breeze(no pun intended).
As frost I do plenty of damage in 5-mans, maybe not as much as I did as fire, but still plenty. Soloing is very easy. Like many others have said, there are several times your enemy will not even reach you, and if they finally do, frost nova and repeat. Did you get a couple adds? Even the score with the Water Elemental. The ice tree just has a lot of things going for it and I'm glad I made the change. Not to mention of course, all those fire resistant monsters I killed in the Burning Steppes as well as the fire immune infernals I killed in Hellfire Penninsula.
Don't worry about your damage. You will do more than enough and you will kill plenty quick enough. I have very limited experience in BG but I know people say ice is the way to go there and it's not hard to imagine why. Ice may not destroy as quick as fire but for reasons already explained, there's many reasons why people choose it to solo.
That all being said, if you're still adamant about leveling as fire and you're
comfortable with it, by all means do so.
Edit: Forgot to mention AoE grinding ;)
Edited, Apr 23rd 2008 4:30pm by Dansmith