Going to be used for car maintenance/restoration work and around the house repairs.
Core functionality of a multimeter:
Measures Voltage - the majority will cope here, as you typically are only dealing with 0 - 12 volts across most areas. You aren't going to need it to measure high tension things such as spark plugs, as you can check them using far simpler methods (such as a spark from engine block to removed spark plug)
Measures Current - currents in automotive circuits and audio circuits can be very high (4 or 5 Amps in a headlight circuit, 100s of Amps to the starter), look for a multimeter with high current capability (2 to 10 Amps isn't too hard to find).
Measures Resistance - you'll probably find the most useful part of this is an audible continuity checker for checking cables deep in the bowels of your car
Frequency - less common on multimeters, but can be useful when working out speed/timing issues
Also make sure it is water resistant, and ideally comes in a rubber housing (as per @CodeBlend - Fluke are generally a robust and accurate brand to go for)
Anything more, and you're likely to need a computer with adapter and software to look at your ECU.
While I've got quite a bit of electronics and mains test gear, I just got what I missed for automotive use - a multimeter with a clip on DC amp function. On vehicles currents are often quite high and it's nearly always very difficult to insert a meter into wires. Fluke would be nice but are expensive, so I got a small Uni-T model. Seems to do the job.
Ohm per volt rating the higher the better, Digital are minimum 1 M ohm per volt -Maximum 10 M ohm per volt. Analogs are in range of k ohm per volt- more loading of the circuit resulting in a lower than actual value. The frequency function probably would be worth not much as tach signal are pulses and would be read better with a scan tool. The frequency function might be good for audio or steady sine wave. AC and DC function for current and voltage. An Ampclamp (essentially a current transformer) would be on my wish list.
I know a couple of bits and pieces but not enough to recomend, here is a forum with some people that sound like they know thier stuf;
I hear Fluke meters are the business. But for 90% of people just go and drop $50 or something at a decent tool store.