Hunting down a series of maddeningly elusive rare spawns will net you this achievement, a nice bit of lucre, and -- if you accidentally kill Loque'nahak while a hunter's camping her spawn elsewhere -- an enemy for life (try not to do this last part). Unlike their classic and Burning Crusade counterparts, who typically drop greens or blues of varying worth, each of the Northrend rare spawns drops gold, cloth, and a bag containing a potentially valuable blue BoE. Assuming you complete the entire achievement by killing all 23 rare spawns, you'll net 345g outright on the low end to 575g on the high end, in addition to whatever else you can make by auctioning the cloth and BoE's.
While hunting down rare spawns in Storm Peaks (and more particularly Vyragosa), you may even hit the motherlode and happen across the Time-Lost Proto Drake, who also drops a bronze proto-drake mount. While the Time-Lost isn't among the criteria for Frostbitten, the mount's certainly a nice incentive toward adding him to the kill list.
The addons NPCScan and SilverDragon are often used by players hellbent on getting Frostbitten finished (although both will require you to clear your Cache folder in the WoW subdirectory if you want to use the addon to find the mob again ingame). Rare Spawn Overlay can also be helpful, but unfortunately it hasn't been updated since June 2009.
100,000 Honorable Kills (or most variations thereof)
Put plainly, there are no immediate rewards for the honor you'll pile up from battleground wins and honorable kills. What makes the various Honorable Kill achievements lucrative is that all that honor you'll be getting from those kills can be converted into gems with Jewelcrafting Quartermasters available to both factions. The Alliance can visit Captain O'Neal in the Champions' Hall in Stormwind; the Horde can visit Lady Palanseer in the Hall of Legends in Orgrimmar.
Both will sell you all of the raw epic gems now available from Northrend (e.g. Cardinal Ruby, Majestic Zircon) at the steep price of 10,000 honor each. However, for anyone grinding honorable kills for the 100,000 Honorable Kills achievement (or anyone who just really likes battlegrounds), getting honor-capped while running out of ways to spend it is pretty routine, and converting honor into gems is a great side income while working towards your "of the Alliance" or "of the Horde" title. On my realm, it's usually possible to make 150-200 gold per raw gem on the auction house, and you're going to get honor-capped many times over while reaching the 25,000, 50,000, and 100,000 Honorable Kill achievements.
Loremaster of Northrend (Horde) and Loremaster of Northrend (Alliance)
With the introduction of the Dungeon Finder, it's become easier than ever to level solely in instances (it's how I've gotten a Restoration shaman from 71 to 79 with just a dungeon or two a day). If you so desire, leveling through dungeons means the entire array of wonderful Northrend quests can be done at 80, rewarding extra gold for your troubles rather than experience. I had saved all of Icecrown questing as a kind of lore treat for my main at 80 (and was not disappointed), and not only did I get the luxury of enjoying the quests' stories without the added pressure of leveling, but I was also rewarded with a nice little pile of gold for my troubles without the need to spend any of it at the class trainer.
Even if you level (or have leveled) mostly by conventional questing, it's still eminently possible to have an entire zone or more's worth of quests still available after you hit 80. You can easily maximize your profits by: a) using the Dungeon Finder to assist leveling, and: b) remaining in one zone until you've exhausted its offerings, and only then moving on to the next.
Something odd happened in Wrath -- not only did fishing get cool, it also started paying (especially if you're an accomplished cook). The Well Fed buff is considered mandatory by min-maxing raiders (particularly by those miserable souls now occupied by Blood-Queen Lanathel..../shudder), and no small number of valuable foodstuffs come directly from Northrend waters. Now, while you can get 1,000 Fish quite easily by just parking yourself at any coastal byway and yanking up whatever unlucky fish happens along, I find that to be both exceptionally boring and a short-minded means of getting the achievement. There may be gold in that there fountain, but there's also an awful lot of it to be had elsewhere.
In general, there's a good market for Fish Feast materials (Musselback Sculpin from Borean Tundra, Glacial Salmon from Grizzly Hills, and Nettlefish from Sholazar Basin -- or all three from Wintergrasp), because it's a convenient mass raid buff but time-consuming for a single person to farm. Check your local AH to see what else is selling well, then fish accordingly. As always, El's Extreme Anglin' is a highly-recommended site for all your fishing needs.
40 Exalted Reputations
I'm not going to lie -- some reputation grinds (I'm looking at you, Shen'dralar and Darkmoon Faire) are very likely to cost you way more than you'll ever get back from them. But, taken as a whole, getting yourself "the Exalted" title is going to be a significant benefit to your coffers. The game doesn't care which reputations you get to exalted so long as you get 40 of them, and that means that a financially-astute player will cannily avoid the more costly among the grinds.
Most of the BC reps are linked to dungeons/heroics or dailies, and Wrath reputations can be done easily through the Dungeon Finder or dailies, so you can reasonably expect to make money from these. However, the most lucrative reputation-related achievements I've had so far have been:
- Hero of the Zandalar Tribe: Most classes can comfortably solo or duo Zul'Gurub at 80, and the place is a treasure trove of drops, rare profession materials, and bijous/coins that you can always sell on the auction house once you're exalted.
- The Argent Dawn: Save yourself some trouble and combine the reputation grind for the Argent Dawn with Stratholme runs for the Deathcharger's Reins. Hopefully, you'll see the mount drop within a reasonable period, but even if you don't you're likely to be swimming in runecloth, drops, and things like Pristine Black Diamonds on a really good run. If you already have the Baron mount, I found Scholomance to be better gold per hour.
- Steamwheedle Cartel: There's no independent achievement linked to the goblin cities, but you'll be running an awful lot of Dire Maul in order to get to exalted with all four. Not only does this put you within a stone's throw of the DIre Maul arena and within easy to range to check on the rare spawns there (Orbs of Deception still sell for a lot if you can manage to get a drop), but you should also be able to loot Knot Thimblejack's Cache nearly every run (barring those on which you've been massively unlucky with a key drop). The cache will give you a number of tailoring and leatherworking materials, and every so often you'll also get rare or epic patterns like Belt of the Archmage. The goblin reps were easily my most pleasant surprise while grinding to "the Exalted."