The Sarnak will be greatly bolstering their numbers, as for the first time they become a playable race. Sarnaks will have their own new starting city located in the islands of the Timorous Deep, and the surrounding areas will support advancement to around level 20. The Kunark mainland is designed for the upper echelon of levels, and the expansion itself includes an increase of the existing level cap from 70 to 80.
Along with the raising of the level cap and new race, other highlights of Rise of Kunark include an increase in the current achievement point limit of 100 to 140, and a new player mount, the Kunarkian Rhinoceros (which for some reason instantly brings to mind Donkey Kong Country and the rampaging Rambi) to traverse the lands upon. There will of course be new dungeons and raid encounters for the high level folks to sink their teeth into, with Veeshan's Peak (making its return from the original EverQuest) for the top raiders.
You can read about the expansion or pre-order the game from Sony's Station Store. Those that pre-order will receive a new pet for their house, a Burynai Seeker. If you wanted to give the game a go before the expansion hits, EQ2's main page includes a download link to a 14-day free trial.