MC can be 5 manned (although you'll need a tank most likely so make that 6) ZG can be for sure, I 2man it for the tiger/raptor mount each week (doesn't drop >_<) polymorph book is also easy to farm and sells for loads on my server(i do it with a feral druid in T6 though so he takes like 500 damage at most a hit :P )
Bwl ... well never tried with less then 15, and even then vael proved to be annoying, remember bosses are still boss level so they do still hurt, and vael is a race against time, naxx you just cannot 5man in my experience you'd have to have a good tank and i doubt he can do it alone.
pvp side: afraid it's a long grind and indeed when wotlk comes out the gear will be replaced: i'd still suggest you get that resilliance though, i personally went outlands in T1/2 gear with the T3 boots, when i dinged 70 i still had my T3 boots, T2 shoulders and helm, and didn't replace those before karazhan drops so it might just be that you do keep some pieces for a good while.also epic flyers aren't that hard to get, do every quest you haven't done yet (it's like 9 to 11G a quest) and do your dailies, while it's boring as hell it's good money and can easy earn you 2000 gold with your toons a day (even though you need about 33000 >_<)