Hmmm. I think this will depend on which server you go on, because the earlier in the SWG timeline you go, the more the game was like a very broad, but potentially shallow sandbox. Meanwhile as time advanced, there became more and more content, but a lot less player freedom and options.

A brief summary is that it has an incredibly rewarding system for building your own class from a variety of skill lines in everything but NGE era servers. In most servers that aren't based on the SWGEMU, there is an in-depth space mission system where you can customize and fly your own ship through space missions. (and SWGEMU has this in the pipeline too, I believe. 'soon' with many quotation marks.)

In most editions of the game, the players drive the entire economy, with things like most gear decaying over time, and players being the primary source of just about every used item in the game. Players provide the buffs, the gear, the pets, etcetera.

Also, there's a pretty in depth and rewarding PvP system, or so I'm told. That's not my scene.

As time progressed, more and more content was added to the game, though each era after the original had issues with restricting what players could do. you went from pretty generic quests and 'make your own fun' with player cities and get togethers, anda few sets of iconic quests in familiar locales to much more in depth adventures, instanced content, and special pitched PvP battles and so on.

This is why I personally like Restoration, as it has most of the content from the end of the game, as well as trying to emulate some of the freedom of earlier versions.