1. It is NOT newbie-friendly. The UI is horrible and anything but intuitive.
2. Lag. Often it’s so bad, you feel like walking knee-deep in mud.
3. Crashes. The client crashes relatively often.
4. Features. Lindenlabs is not very accomodating to changes, even long-standing complaints and issues have not been addressed, and likely will never be addressed either. They value their customers about as much as I value chewing gum stuck to the sole of my shoes.
4a. The search functions suck. They might have been considered groundbreaking sometime in the early 1990s. Now they’re near useless.
4b. The “map” is about as useless as a kid’s drawing to find your way around. Ok, the kids drawing might be more helpful.
4c. Texture/Geometry handling (caching, transfer etc.pp.) is totally ineffective.
4d. Oh forget it. Their feature wishlist is longer than a roll of toilet paper and gets pretty much treated as such.

On the plus side:
1. It’s an awesome place to hold meetings or to just socialize. Not as good as RealLife, but good enough.
2. Creating things. The building interface is simple enough (if kludgy) to build almost anything at will. The scripting language itself is retarded enough that it’s learnable in no time.
3. Because of the “build anything” ability, there are some truly AMAZING places there. From fun games to educational areas, art exhibits to live music (my favorite) it’s all there.
4. People are, in general, very friendly and helpful.
5. You can, indeed, play for free. It doesn’t cost a dime, and there’s enough freebies so one never needs to spend a single cent. If one needs cash, it’s easy enough to get a little bit in exchange for time, too.

http://www.alexa.com/siteinfo/secondlife.com

See for yourself.