The biggest improvement to The Old Republic with Game Update 4.0 is the fact you can now play solo from level one to level 60 by completing your main story missions and a string of planetary story arc.
You can now take any of The Old Republic's NPC companions and assign them as a healer, damage dealer or tank at will. No more teaming up with a character you don't like just because you need a healer; this is now a game about doing the stuff you actually care about, in the form you want. You're even allowed to play new solo-friendly versions of what were once group-only "dungeons."
Bothered by the endless running across vast open spaces that MMOs entail? The Old Republic now features a much less restrictive fast travel system, and the planetary taxi systems are unlocked from the moment you visit a new area, without needing to journey on foot to each new quest hub. If you play it right, there's very little downtime between quests.