Socializing is my favorite part of the game. I have played EQ for about 3 years, and in the course of that time I have met many good people, a few of whom I still talk to through email. I met my RL fiance in EQ, we spent 2 years adventuring together in the game, and that in-game relationship led to our RL romance and eventually we moved in together and plan to marry. Some people don't understand how I could meet someone in a game and end up loving them and trusting them RL, to them I can only smile and know they are not gamers. Because gamers know that the characters are real people and in general most people play their characters as a reflection of their RL personalities. Anyways, in EQ my most enjoyable times were socializing. I was always part of a guild for this reason, to have a constant network of friends and comrades. I led a guild for about 1 year, and it was fairly successful, though small, we were all very good friends. Often in the game I spend time and money helping new players just because I want to.
Leveling gives a sense of accomplishment and advancement, also you can access greater depth in the game the higher level you are which makes your options much more. It is enjoyable for me to group with other players in pursuit of 'xp', because not only do I get a chance to bond/chat with my online friends, I advance my character as well.
I never really 'immerse' myself in the game. I do not role play, and I play my character the same way I act in real life. I am not the type of person who feels comfortable with 'roles' and am best at being myself.
I never really explored. Like I mentioned above, levelling and socializing are what I spend most time on. occasionally I enjoy seeking out a new dungeon or area with friends, and every so often I will spend some time in a new city or hailing NPCs just to see what they say. But I don't do very many quests.