Great concept, poor execution
I have to admit, I loved Second Life... all through the installation process. When it came to actually using it, I realised what a bag of shite it actually is.
The concept of 3D online worlds seems great, in just the same way that the concept of web pages seemed great ten years ago. I remember everyone and his dog creating their own web pages, just because they could. SL seems to be in the same state - people use it just because they can.
However, we've come to learn that websites are only useful if they have a purpose. SL has no purpose. Selling 'virtual goods' is not a purpose - you might as well set up a US$-to-Monopoly-Money exchange and hope to profit from it. Something like WoW has a purpose, gaming, which is why it does well.
I had hoped that SL would be a controlled, limited environment, which allows people to interact in a simple manner (like Facebook), but it's too random and uncontrolled (like MySpace), allows people to do too much, meaning they don't do anything useful or productive. The sheer complexity of the creativity means it's impossible to get involved, unless you're willing to spend every second of your spare time learning how to use their stupid system.