But what i really want to know is what makes this game still playable? Its been out for ages now and there is no multiplayer to it or raiding of that sort. Main story line seems to go by pretty quick.
Does it all end up just doing these dark brotherhood or mage guild stuff?
There's a ton more, but assuming you like this kind of gameplay.
Dungeons, ruins, caves: Just wondering around Skyrim, you never know what you're going to find, whether that be the Frostflow Lighthouse (Definitely check that out) or the Sightless Pit infested with sick, vile falmer. Dwarven Ruins have all sorts of mysterious little things and traps. The things you will find wondering around Skyrim are quite cool.
Factions (as someone mentioned): These are big, as they come with a long story line. If you're into twists and that kind of thing, definitely check out The Dark Brotherhood and Thieves Guild (the ends of these will definitely surprise you). If you're into the whole magic thing, try out the College of Winterhold. If you're the warrior type, the Companions are definitely for you.
Daedric Prince quests: These are my absolute favorite. There are a ton of Daedric Princes, like Molag Bal, Azura, Namira, Dagon, Sheogorath, Sanguine, Nocturnal, Vaermina (spelling), Mephala, Pyrite (spelling), Clavicus Vile, Boethiah, etc. And almost all of those quests are highly interesting, and some of them make you think about the mechanics or real life. Plus, the rewards are usually pretty good (enchanted weapons, armor).
Side quests: Yes, there are a ton of side quests. Some of them trivial, while others are interesting and lead to bigger quests. Try out "Murder on thin ice" or whatever it's called. Takes place in Windhelm I believe. Oh, and do the quest that involves that crazy bitch Potema (started at Solitude).
Collecting a bunch of plops: Like making certain goals, such as collecting all of the shouts, masks, etc.
A few other things: Getting married, adopting kids, building houses (assuming Hearthfire). Becoming thane, etc.
Skills: Have you gotten any of your skills to 100? If you get smithing and enchanting to 100: Enchant some smithing gear using smithing potions. Continue from there, and you will become an all powerful killing machine that all will fear.
Other skills are also fun to get to 100, especially some of the magic skills, because you unlock master spells.
There really is a bunch to do, but it is assuming you like this kind of thing. A lot of it is just exploring.
Oh, and the Autherial Crown is a fun and interesting one. For sure. AND VERY REWARDING.