I did a little web searching for a good summary, but haven't found much yet. Here's a novice's overview, though, which hopefully will help a bit.

I don't think anybody will relate the speaker impedance to the quality of the speaker, so from that standpoint there is no reason to limit your choices based on impedance alone. Also, it is worth noting that in every case, a speaker's impedance is a nominal rating. When you actually hook the speaker up and start feeding it a signal, the impedance will vary with the signal. A nominal 8 ohm speaker might at times dip down to or even below 4 ohms, for example.

The big issue, as you mention, is powering speakers. When designed properly, separate power amps generally can remain stable even with loads that dip as low as 2 ohms or less (which would be necessary when driving a nominal 4 ohm speaker). Receiver amp sections, however, usually are a bit more restricted. Receivers asked to drive a 4 ohm speaker will often experience problems with distortion, overheating, or even tripping overcurrent protection. If you do intend to use a receiver to drive your speakers, you will have better luck connecting it to 8 ohm speakers because of the easier load they represent.

Does that help at all?