More opinions:
1) You will likely see other people, but leveling up in SWG is very much a solo game. From the "Legacy Questline" (main quest chain that gets you to level 40s), to your 10 daily missions from the terminals for bonus exp, to any of the random quests I've run into, none of them are very group-friendly. You can easily get level 90 without ever being offered a group or having a group with anyone besides your droid. A guild with similar interests as yours can certainly change this, though.
2) I don't know much about the new dungeons, though I have heard a little buzz about them. I'm pretty sure they are geared towards high/max level characters, though. Someone else should tell you more cause I don't know enough to really talk on it.
3) There are quests, but not as plentiful or well-designed as you may be used to. Legacy can be thought of as THE quest chain that will get you to the point where you can quest on the expansions' adventure planets, as outside of Legacy there aren't very many develeped quests on the starter/low-level planets.
4) Think you can find out more on the server screen, with average populations and distances listed for each server.
5) There are a few mods you can use, but they are unsupported by SOE to the point where you aren't allowed to talk about them on official forums. (They aren't really "Addons" per se, because they actually modify the game's files to change the interface.) People use them more because they enjoy the different styles/layouts than out of necessity, though. But the customization you can do with the stock interface is good enough for everything I've needed, whereas in WoW it is pretty much an impossibility to play effectively with the default UI.
6) The best thing about SWG is that it is incredibly open-ended and deep. While the devs have provides some linear content, like dungeons and questlines, I don't think they are the real gameplay standouts... The really cool thing is that you don't HAVE to do that stuff in order to enjoy the game to it's fullest. Your goals can be determined by yourself, and not set in stone or laid out in front of you. (Do you want to be an ace pilot for hire? Or are you more interested in forming a guild and building a city? Maybe you just want to be the most popular performer in the local cantina?)
You really have to experience it and decide if the playstyle is a fit for you. It is a complete and total 180 degree turn from the dungeon crawling style of WoW that you sound like you are used to. I think that's the best thing the game has going for it, though, the freedom of playstlyes.