What to look for in a car tire air pump?

Getting an AC compressor just for inflating tires is overkill, the 12 volt ones are fine;however,there are hundreds of uses for compressed air/tools in a garage but you need the knowledge to use/maintain and have the need to use them. We use air tools almost exclusively on aircraft,lots of carpenters use air tools,and of course many mechanics use air tools for working on stuff. Once you get a good compressor you will spend money on tools,little bit of air tool oil,etc IF you want to. Will need to get an inline oil/water separator and an inline pressure regulator is a lot better than ones just on the compressor for fine tuning things like paint guns. So, if you do get an AC one determine how much you will really use it before investing much.

http://www.harborfreight.com/3-gal-13-hp-100-psi-oilless-pancake-air-compressor-60637.html

The bigger the hp the better. I think mine is 1/2 hp. It seems good enough.

I know you said wall socket plug in, but I like this Slime tire inflater. It plugs into the cig lighter, it has flashlight, you set the pressure and it stops when tire gets to that pressure. You do need to have your car running for it to get enough power to work right, and not drain your battery.
Slime 40022.

http://www.autozone.com/air-tools-and-accessories/portable-tire-inflator/slime-12-volt-digital-display-tire-inflator/989586_0_0/?checkfit=true

Oil-less compressors don't last as long. But oil-lubricated compressors require more maintenance since you have to occasionally change the oil in them.

Name brands can be better than the generic brand compressors. I'd look for whatever has a warrantee of some kind.

For filing tires I don't see how the power makes all that much difference. Bigger is just going to be more expensive. It's not likely going to make you wait any sizable amount longer with a smaller compressor unless you have a tire shop and need to change big truck tires or if you need the compressor for something else like a sand-blaster or a paint-sprayer.

Also, assuming you get a compressor with a small tank and not just a tank-less compressor, you can fill up the tank at home and then take the tank to where you need to fill a tire or along with you on four-wheel drive excursions. Of course if the tank is too small it won't fill the tire. If it's too big it'll be hard to carry around.