STO is a seasoned F2P game that really has the model right. As other posters pointed out, you can earn everything in the game by grinding. Yes, this may take a long time, but that's the choice you make. Those of us with more money than time can spend a few $$ to get that new ship. Others can grind and get it later.
Story, Story, Story. STO has tons of story content for the player. There's 6 or 7 story arcs (called seasons, each with a dozen or more episodes) that involve the Klingons, Romulans, Cardassians, Breen, Dominon, Borg, and Undine. There's voice actors from a variety of the series, even including Leonard Nemoy as a narrator. I heard the actress that did Quark's mom in DS9 last night (she voiced a ferengi trader in the gamma quadrant).
Ships - they're all there. Defiant, Voyager (Destiny class, I think), Galaxy, Sovereign classes, and many, many more. Just go look at the website.
Depth. Wow, is there depth. Space and ground combat, crafting, a reputation system, duty and bridge officers to assign (and collect), there are many systems built into STO. In fact, there is so many systems and depth that it will take quite a long time to learn them all, and even longer to perfect them.
STO is build for star trek geeks. This is where I go to be in the star trek universe by commanding my Defiant class ship (modified San Paulo, actually, for us geeks) against waves of Borg ships, or join in on saving civilians from Undine invaders. It's soloable, and has plenty of group content. You can join a large fleet and contribute to a Starbase or go it alone if you like.
Season 8 is just around the corner - free of course. Including an update to the PvP system. Try it out, enjoy the story, become the Captain you always wanted to be, and I'll see you in the Alpha Quadrant.
And to those asking, yes, the steam starter pack is worth the $10.