Well no actually, MC doesn't go "pretty quick" after you've hit Revered. Since you only get rep for boss kills, you get 1050 rep per clear, not counting any reputation bonuses. That means you need 20 clears to hit Exalted from 0/21000 into Revered, which is 20 weeks, which is 5 months.
Similarly, the 3 PVP reps are easy to raise (just PVP) but by no means actually quick.
Going from top to bottom:
Kalu'ak: easy, just do dailies.
Ashtongue Deathsworn: I'm honestly not sure up to where a hunter can solo Black Temple, since I've never tried. Give it a go, worst case you'll get some trash rep; also keep an eye out in trade for random BT PuGs, they happen occasionally (at least, on every server I've been).
Netherwing: do dailies and/or farm Netherwing Eggs. The latter method is faster, particularly now that the area is pretty much deserted, but boooooooring.
Ogri'la: dailies. As I remember it, it'll take a while, but the last time I did that grind was back in TBC.
Scale of Sands: same as with BT, I'm not sure up to where a hunter can solo Hyjal, since I've never tried. It's much easier an instance than BT, though, due to simpler boss mechanics. Try it out or find a Hyjal PuG in trade.
Scryers: probably the easiest if you're willing to just throw gold at the issue.
Brood of Nozdormu: AQ40 is easily soloable up to Twin Emperors by just about anybody, with different classes having a harder time on different bosses. Make sure you loot trash, they can drop an
Ancient Qiraji Artifact that you can turn in for extra rep. Getting the set from the dungeon also gives 500 rep per set piece, if you haven't yet done it. (Can't remember if you can get your set multiple times), as does upgrading your signet ring. Killing bosses in AQ20 also gives rep, and the whole place is quite easy to solo; turning in the Head of Ossirian from the last boss of AQ20 will also give rep if you haven't yet done that.
Darkmoon Faire: you can throw gold at this, but it'll have to be a lot of gold. An alternative is to just advertise that you're willing to pay people some sum of gold (there's a guy on my server currently doing it for 250g) to turn in their DMF trinkets for them. That can be cheaper, but again, a decent amount of gold.
Thorium Brotherhood: much easier than it used to be. They increased the amount of reputation turn-ins give, and just about everybody can solo Molten Core these days so turn-in materials are much cheaper.
And that's all I can see.
Also:
- I'm pretty sure they're changing/fixing United Nations in 4.2 so it no longer requires 55 reputations on a single person. (I might be wrong about that, though.)
- If you have a warcraftpets page, you can link that and I can take a look and see if you're missing some easily acquired or grindable pets. (I'm at 154 pets currently - granted, I've been playing a very long while.)
Good luck!