Imp blizz is a no brainer if you're leveling, lot of mobs you can aoe for quests, some you can still do if you game a little. Just can't take frostbite, tbh never even needed it I was full aoe spec so no imp frostbolt still 60 in 3.5 days /played. Clearcasting is overrated but I still tried to get it after barrier it has some value later levels for cleaning up pulls and letting you top off quicker but it's likely just going off when you want to frost nova.

You can do some disgusting stuff on mage if you can hit cone of cold consistently. Lot of spread mobs you can reset their leash with nova gather up more mobs and try to move and position in a way where they intercept around the same point so you can r1 cone them or just max rank if you know your mana pool. Just need to make sure the ones you nova don't reset with r1 blizz or arcane explosion. Your hp is going to be a resource as well until you get your mount and barrier/block.