Originally there was a a huge amount of hype about the idea of real world businesses opening up an SL presence as something of a "new frontier", as well as vistualised museums, art exhibits, music shows and so on, but these for the most part never got off the ground.
Second Life generally has two sides; the first is an incredibly commercialist (to the point of being fetishistic) sector, where the players that prefer to treat SL as a kind of gigantic virtual dollset can buy clothing, hair, skins etc for their avatars, and has the same kind of draw as The Sims games, except this is real money we're talking about. This in itself gives one type of businesses at least a good way to generate income; 3D content creators.
The other side of SL are the very large groups of people that play the game to indulge in some sort of alternative fantasy. There are massive populations of furries, Goreans, BDSM practitioners and so on, who again are usually quite willing to pay for content for use in role playing in the virtual world.
I originally joined because I was interested in the technology side of things (I read an article somewhere about players using the in-game scripting systems to create 'virus objects' which spread across the world, which I thought was interesting enough to merit checking out) but I stayed because I found I had inadvertently formed a group of friends, and I even ended up getting engaged (in reality) to somebody I met there.
In a sense, Second Life has thrived, but not as a clean, bright new frontier as originally intended, but instead as a kind of collaborative roleplaying framework for people whose tastes are not really catered for anywhere else.