My (humble) advice would be to not let yourself get blinded by the technical specifications. Although they do reveal information about the amp, they will not tell you how it will sound. And if you do want to look at specs, then don't neglect frequency response and damping factor because they tell you more about how an amp behaves than only wattage.
To give my impressions (in order of importance):
- amperage/capacity
the amount of "headroom" an amplifier has. Not only important for low-impedance speakers, but especially for speakers that are classified as "difficult to drive" (and that term usually means low-impedance for certain frequencies only, or erratic impedance).
- damping factor
tells you how firm the amp can grip a speaker ("control"). Higher damping factor means less distortion from speaker design/placement.
- wattage
tell you how loud an amplifier can go. Only marginally important, only tells you whether speakers/amps are in the same league; for example don't connect speakers with 80dB sensitivity to a 15W amp, and don't connect speakers that are rated for 15-75W to a 200W amp.
- frequency response
gives an impression of the overall design quality of an amplifier. Wider frequency response = higher quality ("faster" sound).