A lot of things that I don't like aren't necessarily deal breakers if the rest of the house has enough pluses to make up for it, and/or is priced agressively enough to make it for it. But here are the top negatives for me...
Outside:
Property backs on or is too near a busy street, train tracks or commercial property. I won't even look at a house that's actually on a busy street. That is probably my only true deal killer.
Poor house placement on property. I hate big front yards, especially at the expense of backyard space. Also, if the houses are too close together.
Corner lots
A backyard that doesn't get plenty of sun. I like growing flowers and keeping a small vegetable plot.
Ditto for a backyard with no place to put a garden.
Too many cars parked on the street.
Nearby homes/yards that are not kept up.
Overhead power lines
Barking dogs and other sources of excessive noise
Lack of mature trees in the immediate neighborhood
No sidewalks
Not walking distance to the elementary school, and to some degree Middle and High schools as well (my boy just started kindergarten, there would be a lot of driving in my future since our area doesn't have bus service)
An apartment complex nearby
Poor landscaping-- including badly placed pools/trees, overgrown yards (I'm talking years of neglect) ugly fencing, etc.
Sloped yards, especially when it slopes toward your house and/or half your backyard is too steep to be usable.
No front porch, or equivalent COVERED area. I don't want to stand in the weather when I'm unlocking the door to get in.
Inside:
No natural light in the kitchen. I want a window in there, and I prefer that window to look out on the backyard so I can keep one eye on the kids and the other on my chores.
No sliding doors into the backyard.
A floorplan that doesn't flow well for our lifestyle
A kitchen that is either to small or too big. I want enough room for my stuff, but not so much that I have to walk a mile each day to get dinner on the table. I cook all the time and like an efficient work triangle. I'm extremely picky about kitchens I'd rather have one that needs to be gutted than one that's new but not to my specifications, and they never are.
Not enough closet space
No extra storage area-- whether its a shed, extra space in the garage or an attic.
Large entry areas or any other wasted space like a long and wide hallway.
Cigarette or pet odors.
Ugly wallpaper
Tiled counters
Pedestal sinks in the bathrooms. I need counterspace in there too!
Any strangely small room.
Nasty bathrooms, they give me the willies. Usually cosmetic and cleanliness issues don't bug me, I can see potential, but not in a bathroom. I'm just grossed out.