Updated 11/11/10 with Masith’s comments below.
Something that has not gotten a lot of attention is that there are elementals popping up all over the place, from rifts that appear in certain places. Destroy a rift, get a buff. Get one of each buff, and you get the achievement, [Tripping the Rifts].
If you are level 80, you need to get this in Northrend if you want to grab the daily quest that drops once a day (otherwise, old world will do just fine). Right now (on Alleria), Fire and Water spawn at the bottom of the hour, or up to ten minutes before, and Earth and Air spawn at the top of the hour, up to ten minutes before.
If you time this right, you can get all 4 in an hour.
Here are the spots I camped for mine.
Air: Borean Tundra, just north of Fizzcrank Airstrip, or the captured facility out in the dry lake (just to the west of it).
Earth: Grizzly Hills, just above the cauldron at Zeb’Halak.
Water: Dragonblight, just outside the Ebon Sanctuary cave entrance (just south of Fordragon Hold)
Fire: Zul’Drak, beneath Zeramas.
You have to be fairly close, so don’t quibble to use WoWHead for the exact location.
For level 70 and above, you can also look in Outland, for the daily.
For level 60 and below, it’s very zone-dependent, so look at the maps attached to the above links for more information.
This is likely a limited time offer.
(There was initially some confusion regarding the level. To be clear, you get the buff from any of the rifts. However, the first one each day drops an object that you can turn in as a daily quest. THOSE only drop from level-appropriate rifts. h/t to Masith for the correction.)