I am not infallible. If you have brands you'd like me to add to the lists, or think I made a mistake - leave a comment!While I'm hardly an expert on car stereos (seeing as how I don't own a car), I can at least comment on them from what I've learned over at DIYmobileaudio.com, and hopefully save people from the agonies of a lousy-sounding or overpriced car stereo.
1. Subwoofers are not everything. You will not get better sound quality from a subwoofer, except for at very low frequencies. Strictly speaking, a car subwoofer is unnecessary - some unusually well-built mid-woofer/tweeter combos can do the full range from treble to bass, just like in a home stereo.
Sadly, it's true that it's much harder to get nice perfectly-sized ported cabinets in the kickpanels of your econobox. Bass response is sacrificed for convenience, and a subwoofer is often even more necessary in a car than in a home.
2. Louder is not better. Simply because something is very loud, does not mean it sounds better than is very soft - strictly speaking, a pair of Beyerdynamic headphones put out a lot less noise than do, say, a cheap pair of speakers from RadioShack, but the Beyerdynamics sound a lot better.
Many high-end systems don't use much power at all, and are not really all that loud. A solid 50wpc reciever will go miles farther than 5,000 watts of cheapness.
3. More power is not necessarily better. Amplifiers are measured by distortion specifications, not by how many watts they can put out. In fact, better-quality amplifiers - those heavily biased into class A/B and which inside look oddly similar to a high-end home audio amp - inherently have less power output than a crappily-made monstrousity.
4. Cheap components rarely live up to spec. While a cheap amplifier rated for 3,000 watts might produce that amount of power for a brief moment, that moment would be the tiny fraction of a second while the "magic smoke" is let out. On the other hand, a massively expensive amplifier - say, a Sinfoni - will put out its rated power 24/7 day after day without batting an eyelash.
5. Make sure you have the drivers placed properly. Strictly speaking, the placement of the mid-woofers and tweeters is just as tricky as the design of a standard loudspeaker - except slightly harder, because normal speakers generally give a larger degree of freedom. Badly-placed tweeters and mid-woofers will cause a sonic nightmare.
6. Make sure you have a good install. Improperly-mounted drivers will cause nasty buzzing, rattle, and simply not sound good. In addition, mid-woofer enclosures must be heavily damped to keep your doors from buzzing like bees.
Furthermore, car audio cables, due to the low voltage, must pass a far greater amount of current than normal stereo equipment. If you don't know what you're doing, you'll likely end up with a massive fire. Be very careful of how your equipment is mounted, and make sure that all connections are insulated. Or, better still, have a professional do it.
7. Check your configuration.
There are half a million ways to wire a car stereo - but make sure that you've got an appropriate crossover, or a well-calibrated active crossover. Lower-quality high-power amps go on the subwoofer; higher-quality lower-power amps should go on the mid-tweeters.
Coaxials and "component" (mid + seperate tweeter) sets are both good, but the nature of the crossover can make some better in certian placements. Experiment - and audition - for best results.
8.. This is likely the most important of all: Get "high-end" equipment. While cheap junk might be loud, a quality setup will simply sound better.
Brands to look for:
-Pioneer (For head units)
-Dynaudio (warning - OVERPRICED!)
-JL (I'd avoid the coaxial mid/tweeters, though)
And many more.
Brands to avoid:
Kenwood (except for amps)
Kenford (similar name - cheap crap!)
There are plenty of exceptions to these lists, but you'll be happier with better-quality components, no question. Yes, you'll need to spend a little more - but then again, a professional install's not cheap either!