This rep situation is sort of funky, but we're actually going to have a post later today about how the various reputations work. To hold you over until then: Alliance Vanguard is your 'primary' reputation here. Valiance Expedition, Explorers' League, and the Silver Covenant are secondary reputations. You don't need to directly worry about the secondary reputations, they seem to only exist as a way to raise your primary reputation. They don't really have their own rewards in a regular ol' Quartermaster sense. You want to raise that rep to raise Alliance Vanguard reputation. Alliance Vanguard reputation is also gained through doing level 80 dungeons and heroics without a faction-specific tabard.
The lag in Dalaran is bad, no horrible. It often leads to me being disconnected, it has a carryover effect after I leave Dalaran, and basically makes me very reluctant to go to that great city. Anywhere else in Northrend (that I've visited, at least) is fine. I've never had such problems when Shattrath was the place to be, it's just Dalaran. I expect it to get worse as more and more folks gain access to the city.
Why is it so bad in Dalaran and no where else? And, more importantly, is there anything I can do to help ease the strain? I'm already running MS Media Player, which helps boost my FPS for some reason.
You'd probably get better help going to the official tech support forum to get a little bit closer to a for-sure answer, but one of the first things you should check is RAM. RAM is the most important thing for running WoW, probably. If you have a terrible, horrible video card but have a GB or three of RAM, you'll find the game runs pretty well. Places like Dalaran and Shattrath, areas with high concentrations of players or player-esque NPCs (bare models with armor applied on top, etc, not just baked models like dragons or gorillas) will devour your RAM.
That would be my first and biggest suggestion. I'm running 1GB of RAM, and I am very seriously considering picking up another 1GB stick. Maybe even two. It really makes a huge difference. If that doesn't help, it could be a number of different things and then it's time to call in bigger guns.
Is it worth doing the IQD dailies on the way to lvl 80 if you really really want a mammoth?
No, not really. Not at all. Quest up to level 80, and then do the level 80 daily quests or finish whatever regular Northrend quests you haven't yet. The new content is going to generate you far more money than old content will. Maybe there will be exceptions like gathering skills (Thorium is still worth absurd amounts of gold), but as far as quests go, stick to the Northrend dailies.
Any guesses on how soon we'll need the crafted frost resist sets?
I'm not sure you need them at all. Sapphiron required it in the 40-man version of Naxxramas, but it's certainly not important in the 10-man version. You don't need it, period. Sapphiron has also been done without the Frost Resist in the 25-man version. So how soon will we need it? Maybe when they release that Icecrown raid.
I had a question bout dailies, are there any areas where there are multiple dailies within a zone like there were with the Isle of Quel'thlas? Or are the good old days of Ilse like dailies gone and we have to zone hop to do them?
There are hefty daily hubs, but not early on in the levels. Early on you'll only see a couple in a zone. As you get to higher level zones, they have more and more daily quests. Grizzly Hills has a ton of PvP oriented quests up and down the river in the zone. Icecrown is absolutely packed with daily quests after you've quested through certain areas. Zul'Drak has a series of them that you unlock as well. So far, Icecrown has the most I've seen, but I haven't finished questing in Storm Peaks yet. I imagine Icecrown will remain supreme, though.