The number of things that potentially could go wrong with your John Deere L110 Lawn Tractor is almost infinite, but don't let that stop you from trying to diagnose the problem. All malfunctions will fall into a few simple categories, and troubleshooting to find a cause is not that complicated.
Engine Won’t Start
Don't be too quick to overlook the most obvious causes when the engine won't start. John Deere installed a safety switch in the L110's brake pedal, and the pedal must be pressed down fully before the engine will start. It is also impossible to start the tractor when the PTO switch is turned on, so make sure it's turned off. Open the hood and check to see that the spark plug wires are connected firmly to the plugs. You also might want to install new spark plugs, especially if the old ones are worn or coated with gunk. The fuel filter might need replacing if the old one is clogged. A dead engine might also be a sign of a dead battery--see the "Electrical Problems" section for more information.
Engine Runs Poorly
Poor engine performance may be the sign of a clogged fuel filter, or of old or dirty gasoline. John Deere recommends not using gasoline with alcohol additives. Change your oil and filter at least once a year, or the dirty oil can contribute to poor engine performance. While you're changing filters, don't forget to clean or replace the air filter. It also might be time to replace the spark plugs or check the spark plug gaps. Here's one more obvious problem you might be overlooking: Do you ever lift up slightly off the seat while driving? That would cause the engine to stall, due to the safety kill switch under the seat.
Your battery may need to be recharged, especially if your L110 has been sitting for several months. If recharging doesn't help, you might need to replace the battery. If the battery terminals are corroded, clean them up and use some battery terminal grease to keep the rust away. Check the fuses and replace any blown ones. If the lights don't work, try the obvious fix and replace the light bulbs.
Drive Train Problems
If the tractor vibrates or rattles excessively and if you've tightened all loose bolts, it's probably time to replace the drive belts. Slow acceleration is also a sign of a worn belt. It's good practice to regularly raise the tractor and remove all sticks and weeds that are clogging the pulleys and belts. John Deere's service manual lists a couple obvious reasons for a tractor that won't move: The free-wheeling lever might be in neutral, or the parking brake might be engaged.
Clean out the mower blade area regularly, because the buildup of grass will seriously hinder mowing performance. Bent or damaged blades could cause shaking and shimmying. Only mow at full RPM, because the L110 is not designed to cut grass effectively at too slow of a speed. Remove all debris from the belt and pulley area in order for the blades to work well. Poor tire pressure also can affect mowing performance. Additionally, make sure the blades are sharp and well balanced.