I'm certainly no expert, but I'm willing to offer some explanation.
Speakers can distort the original signal sent to them by the amplifier in a very complex way, and making physical measurements of this distortion can be quite difficult to do. This is why most reviews concentrate on the subjective opinions of the reviewer, rather than doing objective physical tests. These are all subjective descriptions:
Bright - Speaker sounds sharp and crisp.
Warm - Speaker tends to take the edge off music/transients.
Imaging, soundstage - How well the speakers can resolve, in a good recording, the original positions of the instruments in three dimensions.
How do these relate to objective, measurable characteristics of the loudspeaker (like impulse response tests)? The first two subjective descriptions, warm and bright, will be largely affected by the transient response of the speaker. If the speaker has a tendency for allowing high frequency transients to persist longer than they should, then the speaker may sound excessively bright. Another speaker that does not reproduce transients well may have a warm or even dull sound.
As for imaging and soundstage, these characteristics will be affected by how well the speaker can maintain a uniform frequency response over a wide area, i.e. does the speaker sound the same when pointed straight at you as it does when it is angled away from you? The design of the speaker enclosure will have a bearing on this. This dispersivity specification of the loudspeaker is useful here.
To be honest, and I know that this is a sweeping generalisation, most modern loudspeakers from good manufacturers should perform very well. The entirely subjective analysis of speakers performance in almost all reviews I think is quite flawed. I also think that the differences in sound of loudspeakers is often exaggerated, in the sense that most decent quality speakers will offer extremely good performance. Normally weaknesses should only be really apparent at very high volumes.
How the speaker sounds will be drastically affected by where you position them in the listening room and the listening room's acoustics. For example, the soundstage will be improved if you keep the speakers clear of any other objects and maintain a level of symmetry in their positioning.
The Bowers and Wilkins website has an FAQ on speakers that you might find useful. I don't have Flash installed at the moment so I can't give you a direct link, but it's on the site somewhere.
