Consumer Search put together a comprehensive article on buying your new lawn tractor. I feel it is one of the best written articles available to help you decide which rider, lawn, yard, garden, estate, or premium tractor to buy.
I disagree on one point in the article. If you have lots of trees and obstacles ConsumerSearch recomends a zero-turn. If you have less than 3 acres of lawn, I suggest you look at the Craftsman tractors with the 6 inch Turn-Tight Steering first. Residential zero-turns (under $4000) tend to be very hard on your lawn and only work on flat ground. If you have hills, slopes, or no experience using zero-turns stay away from the residential models.
If you have 3 acres or more look at zero-turns in the $4000 price range or higher. Most of them are heavier and can handle some slopes. The Husqvarana 2012 Husqvarana MZ5424S is my choice for the best value zero-turn this year.
When you are done come back here and read my reviews.