Now I've made it safely, I may as well give a rundown on what I found possible and what I chickened out of on the way up - I honestly can't remember Outland much now, that was over a month ago, but Northrend and Cataclysm I can still remember:
Borean Tundra - entered at 70, finished 76 (jumped zone to Howling Fjord partway then came back):
Mobs were hardhitting at first, improved when I levelled up. Skipped the quest leading to the magnataur cave in the north, because it required some crafted item from the AH, and didn't bother going into the cave to find the guy at the bottom who has a quest since, really, I hate magnataur caves!
Skipped Kaw the Mammoth Destroyer; well, I tried it, but ran away when the first 2 mobs took most of my mammoth's health before I even started the elite. Not worth it.
Should have skipped Cracking the Code - it's hard to not aggro multiple mobs there and those casters hit hard. Even at lvl 76, I very nearly died.
Howling Fjord, entered at 71 or 72, left at 75ish:
Easier than Borean Tundra, or so it felt to me, no really tricky quests. The one involving the spirit world made me nervous, since I know if you go too near the Lich King you die, but wowhead pointed out the right building, so I never went near him, phew.
Dragonblight, entered at 76, left at 80:
By this time I was way outlevelling everything so fairly easy.
Secrets of the Scourge - the constantly spawning ghouls were nasty in the extreme, especially with all the mobs behind me respawned, I took the hearthstone route out of there.
Wintergarde Mausoleum is a pain, abominations that a) patrol widely and b) respawn within 10 sec of being killed, makes for a lot of killing abominations and not a lot of progress. Do not recommend unless you're way over lvl for it (or you have stealth)
Grizzly Hills, entered at 79, left at 81:
Still miles over lvl for the zone so again, very little trouble. Skipped the quest involving riding a horse chased by worgen (vehicle quest potentially ending with many mobs on you if vehicle dies), and obviously skipped the one which requires you to die to enter the spirit world. This meant I lost a couple of long chains but couldn't be helped.
Zul'drak, entered at 81, left at 82:
Skipped the Drakuru quest, which I've always had trouble with (in fact, attempted it but he didn't turn aggressive, and decided not to try again: interesting note, you can hearth with a pet scourge troll thing and it'll come to Dalaran with you! Don't think the guards were very impressed
)
Skipped the riding the storm giant to kill elites one; again, a vehicle quest you will die if you fail.
Rest of zone seemed fine, although beware of the enchanted caches in the place with the tiki warriors; the curse of agony wasn't fully absorbed by shadow ward, so would hurt a class without an absorb.
Sholazar, all lvl 82:
Skipped riding the Etymidian, for same reasons as the storm giant in Zul'drak.
Should have skipped Post-Partum Aggression, but just survived, probably my closest call.
Storm Peaks, entered lvl 82, left 83:
Skipped Siriana Iceshriek, elite quest = bad idea.
Had intended to open Sons of Hodir dailies earlier but The Drakkensryd is a bad bad idea for a pet class until you outlvl and outhealth the mobs, so only did it at 83. Was alright then, if a nuisance.
Skipped The Brothers Bronzebeard; another no-pets fight with the added bonus of a huge fall if the plane dies, no thanks.
Found Fervor of the Frostborn kinda scary, it took a lot of kiting! And not realising you have to stand still to cast it let him catch me at one point, luckily sacrifice has its uses.
Skipped The Iron Colossus, another vehicle quest with bad consequences of failure.
Icecrown, all lvl 83:
General idea of this zone: go as far as possible on each chain until you hit a group quest or a nasty vehicle quest: the vehicle quests I skipped were The Sum is Greater Than The Parts, and Army of the Damned, both of which I've failed on regular characters and died as a result. There is a lot of elite quests in this zone so I blocked several chains but compelted 110/140 according to my achievement progress.
Should have skipped Needs More Info, as said above
Hyjal, lvl 83-84:
Named mobs here are nasty. Gar'gol was the worst, struggled badly with the amount of damage he was doing to my voidwalker while he had 75k health.
Sethria was hard as well, but she didn't deal that much damage, it was just she has one heck of a lot of health. Took about 3 minutes to complete the fight, through Bane of Doom and Health funnel, mainly.
Stopped at Cindermaul, since I remembered adds and wasn't sure how safe it'd be (I see all the other 85s must have done it so it probably was ok, but still), so did everything else I could and then was blocked from finishing the zone.
Vash'jir, lvl 84:
Significantly easier than Hyjal, toughest fight was probably Overseer Idra'kess, but a bit of careful pet kiting helped with the fluid slimes exploding to damage him.
Biggest problem was a bug that left me staring at a stone wall instead of being able to go through it to talk to my quest NPC, sigh - that cost me 3 hours waiting for my ticket to be dealt with (only workaround is to die, erm, no thanks!), all while I was on 90% of the lvl and feeling quite impatient
GMs were great when I reached them though, no further problems, since most the tricky fights were in the Battlemaiden chain and she's overpowered anyway.
Didn't finish the zone since I want to try and lvl further in MoP