In Star Trek Online you play a new Star Fleet officer who, due to extreme circumstances, is put in charge of a ship and sent off to do all the usual Star Fleet things: exploring space, talking to aliens and shooting those aliens with your phasers and photon torpedoes.
The game's basic exploration quests come in a couple flavors that you'll pick up pretty rapidly. They can get monotonous but unlike many MMOs you shouldn't need to spend a lot of time grinding. The quests give a lot of exp and simply making my way through the storylines I found myself leveling quite quickly.
New levels means a higher Star Fleet rank and bigger and better ships to command. The game has a class system: Captains are either engineering, tactical, or science officers which have different abilities but all of those abilities can be acquired by having the appropriate classes in your bridge crew.
As far as bells and whistles go, players have the option to create their own alien races in both appearance and racial bonuses. Clothing, ship exterior and ship interior all have some level of customization. It isn't as dandy as City of Heroes but it's enough to give your captain a sense of individuality in the vast mass of officers.
The game also has the prerequisite MMO crafting system which would work well if it wasn't for the large amount of each material required to make anything. The materials can be purchased from other players for relatively cheap and the industrious player can make a profit building ship parts out of their anomalous science but for those who don't want to bother taking the time you aren't missing out on much.
The most fun I had in the game was in the space battles. Shields are directional and weapons all fire in particular angle ranges so your position matters. It gets most interesting toward the end game when you have fleets of federation ships up against those of the Klingons or the Borg.
If I had to settle on downsides to the game, my first complaint would be that the music gets really old, really fast. My other main worry for the game can either be a negative or a positive depending on your play style: Outside of strike forces and and fleet actions which are your major end-game activities, there is no reason to ever involve yourself with your fellow players.
You can cooperate with other captains on missions but in true star trek fashion you tend to bring your whole bridge crew down on away missions so there is never a real worry of being alone against your enemies. And while some of the space fights can be challenging, it was usually a problem that could be solved by upgrading my phaser banks and torpedoes instead of trying to call in friends for help.
Over all, I would consider Star Trek Online worth a play for certain if you were a big Trekkie and if not it is still a good game especially by standards of what you can play for free.