I find myself entirely amazed at the way that ESO instances the content you see without placing you in an instanced map. You can run around in the same area as other players and while they're seeing hostile enemies all around and battling their way from building to building, you might be finished with the quests in that area and surrounded by friendly NPCs instead. And it all blends together seamlessly. It blows my mind.
Running in dungeons is interesting because whether you're on a team or not, there doesn't seem to be any significant difference in reward for defeating enemies solo or with the assistance of another player. The only difference seems to be how fast the enemies are defeated. You still have the same chance at loot drops from the bodies. I can't really say what the difference in XP is (if there is one) because I never really watched that closely. If there is a difference, it didn't feel like it changed the speed at which I was progressing through the game.
The biggest draw for me, though, is that I can solo to my heart's content. (I know, I know, there's that whole "Why would you play an MMO if you want to solo?" mindset, but I just like to play games and be left alone for the most part.) I have one character I've been playing since the first free-to-play anniversary weekend that's at level 50 and somewhere around 240 champion points beyond that, and I've never teamed with anyone. And I still have over half of the zones in the game left to explore, so there's plenty of content still waiting for me. ESO is probably the most casual-friendly game I've seen since CoH. I love that.