after trying the open beta i personally like it even though its not that dissimilar from other MMOs of, "kill X or fetch X" for one difference though and i'm sure you've heard it before already; the combat system. now while it might be a moot point for those that didn't try your list (Dragonsoul, DDO and Spellborn) Tera is a refreshing breath of fresh air for someone whose had nothing to do but WoW for the last 8 years >,<. now don't get me wrong WoW is still a good game but as we all know its old and getting stagnant; the same old, "auto target and spam ability" has really destroyed my interest in the game. I gave SWToR a try and while better is still basically a, "WoW in outspace" system that while impressive I still found uninteresting.
now the combat system around Tera is nothing new but it is for a majority of people who have tried your list OP but wasn't impressed by it (tried my hand at DDO and even the starting area was a snore for me). Different strokes for different folks I say, me i'm having fun with Tera Online and i'm gonna go with it. my rule book is, "if i find something entertaining i play it, if I don't find it entertaining I chuck it," which is the reason WoW and SwTOR are taking a back seat. True I might grow bored of Tera someday thats a given but for now i'm having a blast with my slayer =D