When replacing speakers I looked at 4 specifications: impedance, power, frequency range, and sensitivity.
Impedance is electrical resistance measured in ohms. I think it is best to match the impedance rating of the speaker with the rated capability of the amp. Stock speakers in my UHFS system are subwoofer - 1 ohm, front door - 1 ohm, rear door - 2 ohms, mids - 2 ohm, tweeters - 4 ohms. If speaker ohms are numerically more than amp channel rated ohms then the volume will be less by a few decibels than if the speaker impedance more closely matches the amp channel rating.
Power ratings for speakers are measured in watts and there are typically two specs - RMS max which is for continuous, use and peak max which is measured at a point in time. I think speakers should be rated to handle the rated power output of the amp channel that powers them.
Frequency range is the measurement of the low and high frequency rating of the speaker measured in hertz. Depending on the type of speaker and intended use, the ranges vary. Most humans hear somewhere between 20hz and 20,000hz, but this varies by individual. Subs, for example, sometimes range from 40-400hz, but different brands and models vary. Tweeters, at the other end of the spectrum, sometimes start around 3,000 or 4,000hz and go to 20,000 or 25,000, but again, this varies. The stock Model S tweeters have a capacitor attached that low filters frequencies lower than 4,000hz (to protect the speaker and help prevent distortion).
Sensitivity is measured in decibels and indicates the volume level the speaker should be able to handle with reasonable clarity. I think 90db and above is a good sensitivity level for a speaker, but many speakers are rated a few decibels below that and are decent, in my opinion.
When selecting a speaker, I take all of these into consideration, in addition to price, and I look at manufacturer frequency distribution charts to see drop-offs and impedance ratings at various frequencies.
It all boils down to a few real-world factors for me - what are the best overall rated speakers that best match the amp and amp channel that will power them? what can I afford? what are my music and sound preferences? what am I comfortable installing myself or having someone else install? and perhaps most important - what do others with real and relevant experience (not just speculation) say about it.