The Jedi and Sith Classes will be similar to what they were in KOTOR and TSL, but probably more extreme. A Guardian/Marauder will be even more lightsaber and brute power focused, however. Consular/Lord will be the usual mage class. Also, this time we will no longer have the Sentinel/Assassin class (both pointless middle ground classes who do very little thing differently from non-Force users).
Republic Troopers and Sith Legionnaires will essentially just be the same class (ranged front lines soldiers) but different allegiances. Smuggler and Bounty Hunter classes are essentially the Freelancer classes, but aligned respectively: one makes money by selling goods illegally, the other makes money by killing or capturing people.
As of 3/14/10, TSL is restored. The Sith Lords Restored Content Modification by Stoney and Zbyl has been finished and can be downloaded here