1. Bad Smells

I have been in a number of houses that have big offensive odors. It is a huge turnoff for buyers. These smells can range from cooking odors (curry, or fish), to cat urine, to cigarettes. Sellers often can become immune to their own homes scents.
To rid your house of smells may not be easy. Sometimes it will be enough to change out the carpeting and to wash the walls. But sometimes it'll need to go as far as painting the walls and ceilings with odor-blocking paint such as Kilz Max.
I once walked into a kitchen that had a whole pig in the process of being butchered right there on the center island. Believe me that the buyer wasn't impressed.

2. Pets

Dogs that are not kenneled can be an issue. It is not always easy to have a dog jumping up on people when the buyers or the buyers agent may not like dogs. Or it can also be a huge distraction if they are kenneled and barking like crazy. If they are quiet dogs, kenneling them might be fine. If they are very tame and just lay around, it may be ok for them to be out and about. But keep in mind that some small kids may get scared of a large dog that is not kenneled.
Cats are usually much less of a problem, except when it comes to the litter box. Be sure to keep the litter box very clean so that the urine smell doesn't emanate throughout the house.

3. Clutter

I have been in homes that were so messy it was nearly impossible to get across the room. I also have seen cash laying out on the counters. What happens if cash disappears from a home during the day, and there were several showings? Did someone take it? Don't leave cash and personal information (bills and other documents) out for anyone to see.

4. Homeowners

I have come across homeowners in several houses. In one house, I knocked, opened the door, and announced myself and the young female taking a shower said "just come in and look around." A short time later she was walking out of the bathroom in a towel. Awkward.
In another house, I again knocked, opened the door, and announced myself, and there were 8 large men sleeping in the living room, some on couches, some on the floor. In this case, I just slowly backed out and closed the door. We didn't even see the rest of the house.
In other situations, homeowners were just leaving out the side door as we arrived. That's fine.

5. Hard To Get Into The House

Sellers should make it easy to get into the house. Lockbox should be on the front door for 2 reasons: 1. the first impression should be the living room, not the side entry to the mudroom. 2. In winter, it's more difficult to walk through snow to the side door, and then find a place to put your snow covered boots.
Door locks should function like butter. If not, get them adjusted, or keys recut, or replaced. All locks should be keyed the same, this includes the garage. This way the lockbox should only need to have 1 key in it.
There should be ample room for 2-5 people walking in the front door, and place to put boots or shoes.

6. Alarm Systems

These should always be turned off, and have the chimes off as well. That way nobody thinks there is an alarm system going off. I once walked in a house that had an alarm system, but I was not notified that there would be such a system, nor what the code was. Yes it did go off, and then the house phone started ringing (the alarm company verification call). It was a hassle to call the listing agent, and get the code from her and enter it with 90 db of sirens going off.

7. Inability to easily turn on lights

There are lots of homes that have switches right by the door that appear to do nothing. In Minnesota winters, it starts getting dark at 4pm. By 5pm it's pitch dark, or almost pitch dark. If the switch at the door doesn't turn on an overhead light, make sure it turns on a lamp. Or alternatively, have a light on a timer so that it turns on right when it starts getting dark.