Lots of changes. TSS was / is a great expansion, with new race, and leveling from 1-75+ all in the same expansion, plus lots of good quests, and items, and the new start city of Crescent Reach.
Only big, global thing I remember on TBS was New Tradeskill Trophies. This may have been when the new Jewelry was implemented, can't remember for sure on that one, since it was done in multiple stages.
SoF added Heroic Stats to items, and (actually between SoF and SoD) new sets of globally dropped armor and weapons, called Defiant. Also a revamping of Cultural Armor, and post release, announcement of no new, planned developments in Cultural Armor.
SoD added Mercenaries as the big thing. Tradesill-wise added some nice player-made weapons.
There's a ton more changes, and too many to list in a single post, so have a look at the patch notes:
Seed of Destruction Patch Notes 10/21/08
Secrets of Faydwer Patch Notes 11/13/07
The Buried Sea Patch Notes 2/13/07
The Serpent's Spine Patch Notes 9/19/06
**** EDIT ****
Bah! Forgot the most important resource, read the stickied threads .. New / Returning Players, and Useful Threads.
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Yther Ore.
Edited, Apr 5th 2009 3:35am by Yther