1) Overdone, inked panel lines.
Not as bad as trench like panel lines
2) No pilot/crew figures.
If they were all like the nine o nine crew fine but the 1:32 Corgi Spit & Hurri piolts YUCK!!
3) Poorly designed canopy with glaring gaps, overly thick frames, etc.
A turn off
4) Difficult to install and easy to lose "fiddly bits".
Not an issue for me
5) A basic lack of detail on the model.
If its the only model of its type (IXO UHU) I will buy it
6) No stand or a lousy stand.
Not an issue for me
7) Wheels-up options, without the appropriate part for the tail wheel( Dragon's FW-190s ).
Irritating but not a deal breaker
8) Missing parts like pitot heads, bombs, etc.
As seven
9) Obvious gaps where key pieces of the model join together.
Number one sin.
10)Attempts to pass off one mark of a plane for another ( Witty's "FW-190A-5" which is actually an A-8 model )
It would depend on what it was meant to represent
11)Poor mottling, bad paint job, wrong colors, etc.
I usualy pass on these
12)Hard to justify prices'
I have learned my lesson on this subject (Corgi Vampire 
)
I'm sure there are more deal breaking issues, but this should give us a start.