The box store or small Craftsman LT's are fine for a half or one acre yards. Most people I see here who start off with an entry level LT for more acreage than that usually upgrade to something bigger and better very quickly.
Expensive garden tractors, just like most expensive cars, have a good amount of depreciation after the first year or two. This makes it fairly easy to pick up an excellent used high end GT with low hours when it is a year or two old. A lot of people sell these tractors simply because they move to less property, or they need to settle estates or whatever. New is always great with the warranty, but if you are even slightly mechanically motivated to be able to change oil, grease fittings, and change belts and the such, there are good deals to be had on used high end GT's- and for the most part, they are far superior to any new lawn tractor out there.
For the light LT user, like my mom's Craftsman, (42" 16 HP Kohler hydro) a little care can go a long way. I maintain it for her at the beginning and the end of the season, and its made it through year 9 with no trouble. To this day I've only replaced blades twice, a couple of blade brake springs, and the battery once. Can't beat that for what was a $1100 tractor used on a half acre.
-Fordlords-