I believe Frostfire bolt is more of an ability for mages that do want to spec elemental and get the benefits of both trees into one spell, vs one or another. It's purely a pvp 3 second spell. I see no real viability of this spell in PvE.
On another note, Mirror image is super broken, it does retarded damage and it's gonna get a good hard swing from the nerf bat. Currently, it mirrors a % of your current mana on cast and gives it to the image. For example, say the spell only gives 30% of whatever mana you have to each copy. If you have 300 mana, then each copy will only have 100 mana and the spell's useless. It's better for opening dps increase (as it currently stands, I believe this will serve more as a pvp spell. However, for the cost *360 mana I believe* it would suffice as a decent dps increase for almost no penalty.)
To give you an idea of how OP this is, I can cast mirror image at the start of a duel and pretty much just Ice block and let the images win for me. I've seen them throw out over 15-20k damage in about 6 seconds, which is silly. Not as silly as rogues are right now with their easily attainable 4k dps, but still silly nonetheless.
Also, it seems to appear that Hot Streak has an internal cooldown, making chain crits almost useless. I also haven't been able to tell how critting with a pyroblast affects the talent, if it even does at all.
I ran Naxx 10 man last night for a little bit into the early morning. The trash is pretty ridiculously easy, but the bosses aren't so forgiving. While you're decked out in PvP gear arena season 1 and honor season 1, it's not PvE gear! Warriors with 8% dodge n *** aren't going to fly far, but you make do with what you got. We downed instructor and cleared trash from 3 wings and got about 4-5 BoE epic drops, which are decent in and of themselves. ALSO, a bit of a warning, Anub'Rekhan is going to be the nightbane of Naxx, it's not an easy fight (for anyone that remembers the joy of the spider wing in Pre-BC, it was like being shanked repeatedly with an olive fork. I hated that encounter)
Also, it appears that Patchwerk is a weee bit strong. He hits pretty hard.... Like, a lot hard. And attacks very fast. Basically, I'm going to be running it just to clear trash and some of the easier bosses for some PvE loot to make the other encounters easier.