Large amounts of small denominations is better.
Build Quality - visible seams? glue on glass? wonky landing gear, ordnance? wings/tail unsymmetrical?
Paint - runs, brush marks, overspray?
Decals - crooked, unsymmetrical, upside down? silvering?
In the end, after you've looked at the above, what's your overall impression of the model? Do you like it or not (though your overall views of the real subject shouldn't play into it - Terry WILL put a Wildcat in first place if he's honest!)