Structural Problems - Look for un-level floors, cracked drywall or other un-level surfaces as well as bowing or cracked walls. These can all be possible signs of a structural or foundation problem. These may or may not need to be addressed right away, but might definitely help with negotiations.
Pest Problems - Pest problems can happen. But look for termite mud tubes and other signs of some sort of infestation. These can be dealt with, but can be costly. Good to know what you are dealing with right away and figure out which way you want to go.
Random Fresh Paint - Overall a newly painted room can be nice. However, small random spots of new paint on a ceiling or a way can indicate something that the seller may be trying to hide. It might not be that, but, it's good to be thinking about that so you can ask the right questions before you get too far into the transaction.
Odors - Bad odors can be an obvious sign that something is off. However, an overwhelming "good smell" may be a sign that someone is trying to cover up a bad smell! Try to see the home when there is no extra air freshening things on so you can see if anything is up.
Poor Neighborhood Condition - Location can be everything when you are selling a home, so you need to make sure that the neighborhood around the home you are looking at is good in case you would have to re sell your home in the future.
Stains on Walls and Ceilings - Sometimes a stain on a wall or ceiling can indicate a leak somewhere that is current damage or maybe past damage to a roof or other part of the home. Make sure to pay attention and have those areas checked out as well. You want to know what kind of damage you may be looking at before you purchase the home.
Grading - Always check the grading around a home. The ground needs to slope away from the home so that when it rains water doesn't pool around the foundation of the home causing damage and leaks in the basement.