Once you have passed the point of scurrying around, discovering awesome builds and visiting the glamor and glint fun; once you slow down and start meeting the people in various places in a mood of curiosity and expansive time, then you start to find the communities in SL.

Second Life has to be a sociologist's dream. The communities that self-organize and grow there are incredible in their diversity, accomplishments, wealth and some in their drama. In SL people have a chance to bond around things that connect with them deeply for whatever reason.

AN INCOMPLETE LIST OF SL COMMUNITIES
Here is a list of deep, established communities in Second Life. Some are easy to get into, some are not. Some are in turmoil and change, some are not. However, all of these have a group of people that are dedicated, regular and active members, and all have land, buildings, activities, history, tradition, beliefs, practices, protocol and interaction. They are usually friendly, especially if you show interest and affinity for their culture and premises.

ONE AT A TIME, OLD BOY!
Explore, learn, discover and experience. Just don't join five or ten of these at once. Each one of these kinds of community can take months and many online hours to really participate and get the richness and depth. Being in one at a time is a much better plan. Visit any and all you like; but keep your capacity available to be able to contribute to the one(s) that speaks to your heart and mind.

NOTE: This is hardly an extensive list! I add to this list as I explore and find committed communities in-world, which takes time. If there's a community you'd like to profile on SL Trip Tips, let me know!