I've seen alot of people asking about the viablility of flying different ship types as opposed to their Captains (example, Tac officer in a science ship) and I just wanted to chime in with my take on it, hopefully someone out there will get some use out of it.
Now, seeing as how WoW is the only other mmo I've played, I'll use it to draw some comparisons.
As far as I know, the only real difference between the ship classes is the number and type of bridge officer slots. An escort will have more Tactical stations, a Science vessel will have more science stations, etc... The "class" you play will largely depnd on your officer/ship type combination.
For example, Tac officers are all about the combat, as are tactical ships like escorts.A tac officer in an escort is designed for max damage output, but not very high on survivablility.(glass cannons)In WoW, this would be considered a mage, or possibly a fury specced warrior. Cruisers put out good damage, but are more geared towards defense. This would be like a protection specced Warrior. Similarily, a Tac officer in a science ship could be compared to a Shadow Priest (or a science officer in an escort).
Disclaimer: This is an extremely basic way of comparing things, and I don't pretend to know everything about the game, but I think for alot of people coming from WoW it might help with some career/ship choices. Anyone out there with more knowledge, please feel free to comment or correct me as needed!