SWG was, for me, both incredible and terrible at the same time.
The good:
Community - in no other MMO I've ever tried, has there been social hubs in the same sense as in SWG, giving the opportunity to really get to know people.
Personalization - You had so much control over your appearance that it was actually possible to recognize some people, not by their name plate, but by their avatar's look. Armors were the same, as you could mix and match from several different armor sets and most armors had 10 different slots (head, chest, belt, legs, feet, 2x biceps, 2x bracers, hands), giving you the chance to look unique beyond a a per mil of the population.
I for one had an armor/color combination, that I never saw anyone else have. You could, of course, just go with storm trooper armor etc. as well.
Space - A built in space simulator with free flight, 40-50 different ships to choose from across 3 factions and the ability to reverse engineer looted ship parts. Oh, and you could hook up your joystick to fly of course.
Neutral faction - Don't want to tie yourself to a fighting side? Sure, stay neutral.
Player houses/cities/ships: The ability to place houses, make cities and decorate the interior of your houses and the large ships. Decoration was 100% sandbox using the countless items in game, so you could come across some amazing works.
Species/species traits: Unlike SWTOR, you could choose to play as less humanoid races, such as Wookiee, Trandoshan, Bothan, Mon Calamari, Ithorian, Rodian, and they had different starting stats and certain traits, as well as their own language, which you couldn't understand or speak, if you weren't that race and hadn't learned the language.
I remember my first encounter with a wookiee, he said something, and I was all "Huh?", was quite funny when I thought back on it when I found out about different languages. Languages and traits were all removed with the NGE though.
PvP - Open world PvP, that could happen anywhere, at anytime. Sometimes, all it took was 2 opposing overt players who started fighting, then more players would join, and it could grow into this massive battle going on for hours! There was no real point it in, other than true and utter joy.
Crafting/entertaining - The mere thought of a non combat class is ridiculous. For most games that is. In SWG it actually worked, and it worked damn good.
Fast(er) swoops/speeders - Acquirable already at lvl 1, and at no great cost.
Emotes and animations - SWG had more of this than any other MMO.
The bad:
NGE - I honestly welcomed NGE when it came, as it provided a more action paced combat that I liked. Oh, how quickly what little love I had for it turned into complete hate. So much content removed, profession/skills dumbed down, so many things that got pointless. When people say NGE is the most stupid thing to happen in the history of MMO's you won't get any arguement from me.
Lack of content - There were actually lots of content, but much of it was broken, especially after NGE, and the devs were slow to fix it, if they did at all. Not to mention, even it it was fixed it was still old content, which most players were sick of, and new content was scarcely implemented.
Constant profession imballance - It almost felt like the devs didn't want to fix it, overbuffing/overnerfing everything, but at least that way they could provide a constant improvement plan and make people hope/long for their profession's overhaul.
When all is said however, SWG was the game we loved to hate, or hated to love.
For me though, and for better or worse, in SWG, it felt like you had a 2nd life, you actually lived the saga of Star Wars. Just imagine taking your speeder across the dunes of Tatooine, passing the great pit of carkoon and Jabba the Hutt's palace. Or launching your ship, making the jump to hyperspace and land on the next planet, SWTOR may have this as well, but strangely enough, even beyond free flight, SWG just made it feel more as it should.
Lots of text, but I do feel strongly for what SWG once was.
The basic core of SWG, is quite honestly what EVERY MMO, should have as a base, and build from.