...actually, I don't really like that idea so I'm going to answer your questions like I normally do, starting with Naraxus's question...
Something I was thinking about was how much of an important stat "Hit" was on endgame gear in BC. A couple of patches ago a lot of the lower level gear was given a hit stat to it and I was wondering if the 70-79 gear in WotLK has "Hit" or is it just basic stats and spell power, attack power etc?
There's some Hit on leveling gear, but not enough to keep you capped right to 80 in my experience. I actually didn't really try, I concerned myself more with survival and longevity. In the case of a Priest, that meant stamina, spirit, and spell damage. Leveling gear is a bit more simplified. Raw stats, and some attack power/spell power in most cases.
You'll also find some crit, hit, and haste, but your leveling gear isn't going to have as many random stats slapped on it as you see on some raid gear.
Regarding Heirloom items... Can you put enchantments/enhancements on them? If so, can you transfer those buffs between characters? Seems a little too good to be true to have my level 1 alt starting out with a pair of shoulders with Aldor inscriptions on them.
As far as regular enchants go, like readers mentioned, heirloom items are always considered level 1. You can't put high end enchants on them at all. As for Aldor/Scryer shoulder enchants, I don't know and I haven't earned enough badges on the beta to buy a pair of shoulders and check. I'm willing to bet that if you currently can apply those to heirloom items, the devs will nerf it pretty quick.
Are there class trainers in Northrend or do we have to go back to our Horde/Alliance cities to train new spells?
Dalaran houses Mage and I think Warlock class trainers since it's sort of the Mage capital city. And Warlocks like to pretend they're Mages unless you compare them to Mages or suggest that they're at all like Mages and vice versa. Everyone else will need to go back to the mainland to train, whether it be by boat or by portal.
And that was a joke, Warlocks. Or Mages. Or whichever one of you was offended this time. I don't even know anymore.
Regarding gear, what sort of HP will a lets say level 80 Naxx epics geared tank have? Because I heard people say the entry level raid in Wrath should get you around 20k-ish, is that true?
It's true, and possibly a little on the low side. That's around what Sunwell tanks have right now. After finishing Naxxramas up, they'll definitely have more than what they started with.
Regarding the phasing mechanic, if a Guildie of mine completes Quest X, which, through Phasing, alters the way in which he sees the game world and I have not yet completed this Quest, will we be unable to do anything together anywhere in the game, because the way in which we're seeing the world is different, or will we be able to interact as normal? And if our interaction is limited, how so? Game-wide, Zone-wide, etc.?
It's limited to subzones, as others mentioned. Let's just use Shattrath as an example since it's an area that people of both factions will be familiar with. If you and your friend were doing quests up on Scryer's Tier, but your friend was a few steps ahead of you, he would be in a different phase of the Tier. But if the two of you went down to the Lower City, you would see eachother just fine. If you went up on Aldor Rise, you'd see eachother fine. If you went back to the Scryer's Tier, you'd be divided into your respective phases. He could then wait, let you catch up, and you'd both be in the same Scryer's Tier phase again.