+1 to Manaki. 70 bracket is not very good balanced, but if you got hands/brain you will have high rates even without engineering. Yes, its all about RMP with tazik/synapse, but even with that it can be countered easily if you know what to do; i would recommend you to go gnome to get 3 tinkers at 465 engi ( others get 1 at 450 skill), hope for tazik/synapse, if not- relevel engi again, then go Faction Change to dwarf with frostres racial and get Leatherworking with frostres wrist enchant, +20 on head and +20 on cloak- and mages will not hurt you much.
3 classes that are very weak on 70 unless you got sick skillz : Warrior, Hunter, Warlock.
80 bracket is filled with 84s and that is a very big problem because 80-84 stats are "compressed" and its like pvping lvl 50 vs lvl 60 or something like that- ppl can global you, and on arena two 84s with any burst abilities can 1shot everything- hunters with kill shot, 2 ret paladins , frostmages etc. Queue times are long, there is no lvl80 twink alterac, usually its WSG only.
The most powerfull twink atm is lvl 79- you can queue for bg and leave before it ends without getting alot of XP ( or just pay 10g to freeze your XP after accepting the bg pop, you will have 0.5sec to do that, accepting BG macro + clicking on XP freeze -- yay i just spoiled the biggest twink bug). 75-79 leveling bracket is filled with a4 with no enchant/socket noobs that can be 1shotted- engineering(30k heal, 18k shield) + LW (stamina) are your friends.