14: Arcane Explosion.  Now you can AoE
16: Flamestrike.  A slight improvement to your AoE rotation
20: Blizzard.  You learn it at 20 but you can't really use it yet.
22: 3/3 Improved Blizzard and Rank 2 AE.  A great level!
23: Cold Snap (talent).  Use it to re-nova when you get resisted.
26: Cone of Cold.  Makes your life a lot easier.  Or not, since it's less reliable in Classic.
​28: Mana Agate.  First mana gem.  Yay!
30: Ice Block.  Not a big deal, but nice to have.
40: Ice Barrier and a Mount.  Easy mode.

You learn a new rank of an AoE spell every 2 levels, so each one gets updated every 8 levels.
14: AE
16: Flamestrike
18: Nothing (would be CoC)
20: Blizzard
22: AE
24: Flamestrike
26: CoC
28: Blizzard
etc.