It had an immaculate launch with outstanding graphics, good performance, customizable UI, and innovative features such as the class/talent system. Not to mention the combat was smooth and seamless.
Their world pvp wasn't just confined to one zone like Illum. Their pvp dailies rotated randomly across the uninstanced world.
Their dungeons and raids were complete, unbroken, and dropped appropriate loot.
It's only drawbacks were the complex macro system, which allowed classes to weave 5 abilities into 1 button, removing player skill. Besides this, I don't know why it wasn't more successful. I think it was just bad timing and greedy players. If it were to be released now, in wake of SWTOR's flaws, I believe it would be considered quite refreshing.