Having a good stereo is an absolute must-have in my cars. I can't stand listening to garbage factory systems; it totally ruins thelistening experience for me. All of my cars have upgraded stereos (except the wife's car, which has a pretty decent factory system). Nothing too crazy, just an aftermarket head unit, nice speakers, and a single 10" subwoofer. Another must-have is a USB input so I can plug in my iPod and control it from the head unit, as that is all I listen to. I also really like having Bluetooth. The head units I installed in my Bronco and CR-V have Bluetooth. If/when I upgrade the units in the Accord and Ranger again, they'll definitely get Bluetooth.
The Accord has a Sony head unit, Polk Audio 5" front speakers, Pioneer 6x9s in the rear, Sony 600W RMS mono subwoofer amp and an Alpine 10" DVC 500w sub in a sealed box
The Ranger has a Sony head unit, Hifonics 5 1/4" front speakers, Pioneer 5 1/4" rear speakers, Pioneer 2 channel amp bridged to 1 channel, powering a Pioneer shallow-mount sub mounted in a tiny sealed box (more than enough for the truck's small cab, and space efficiency is a must in there.)
The Bronco has a JVC head unit with Polk Audio speakers (6.5" front, 5x7" rear). No amp or sub yet, but it will eventually.
The CR-V has a Pioneer head unit, and I'll be replacing the god-awful factory speakers hopefully in the next month or so. I have my eye on Polk Audio component speakers for the front, and Polk coaxials for the rear. I won't bother with a sub in this one, as I need the cargo room and this is just my winter beater.