Most people ditch their walk-behind mower because they are physically unable or down-right unwilling to push it anymore.
Riding lawn mowers come in three styles: rear engine riding mowers, lawn & garden tractors and zero turning radius (ZTR) lawn mowers.
They all have different attributes so pick the one that works best for your lawn:
Rear Engine Riding Mowers

If you want a relaxing, easy, comfortable mowing experience, then consider a
rear engine rider. These mowers do one thing, and do it well... cut grass.
Due to their smaller cutting decks, rear riders are ideal for yards with narrow passes like fence gates. Their low center of gravity makes them very stable while mowing on hills.
Rear engine riding mowers aren't powerful enough to pull heavy attachments. Also, they won't break any land-speed records at 5 mph.
Lawn & Garden Tractors

If you want more than just mowing, then
lawn tractors and
garden tractors are for you.
These tractors are built to be work-horses all over your property and haul heavy attachments. Equipped with powerful engines, they have the muscle to accomplish big jobs.
These badboys are like linemen on a football team. Big and strong, but not the fastest, most agile players. If you're looking for high-speeds or your yard has lots of obstacles, tractors aren't your best bet.
Zero Turn Mowers

If tractors are linemen, then
zero turn mowers are the running backs and receivers on the team. These high-performance mowers zip around your lawn at 8 mph and can turn on a dime.
Their revolutionary steering system allows you to make nimble 360 degree turns with ease. Zero turn mowers will save you the most time while mowing.
ZTR mowers do not handle well on slopes and can slide because they do not have large, gripping tires.