To Make A Long Story Short...
There are several factors, depending upon whether it is a full-sized tower, bookshelf, center or surround speaker, that drive my own (or others) decision making.
For a full range speaker (tower or large bookshelf), I'm looking for solid performance from high, to mid, to low range (~30Hz-20KHz) frequencies while remaining as close to neutral from a sonic characteristic aspect, the less coloration, the better. This is not to say that it should be "laid back" or "warm", but should add little to no tonal qualities to the signal. In fact, all of the speakers (not just towers) should maintain this suggested sonic "neutrality".
For a bookshelf speaker, solid performance from a high to midrange frequency level which does not attempt to overstep its sonic capabilities. Highs should be crisp and mids should be tight and solid in attack, but again, keeping the bass response in check so as to not even attempt to reproduce low frequency signals at reference levels. As bookshelf speakers are quite often an alternative to towers due to size restrictions, the aesthetics aspect of design would drive a large portion of prospective buyer's decisions.
Center channel speakers should very closely timbre match a few tower and bookshelf models at similar levels, this would be my biggest peeve when performing listening evaluations. In addition to this, being the focal point of most movie soundtracks, they should be able to reproduce solid low to high midrange as well as higher frequencies. Lack of low midrange frequencies tend to spoil baritone dialogue giving it a far more sibilant ring.
Surround speakers should provide a broad sound field in any setting from a small to large listening environment, minimizing the need for focal point seating positions of the listeners. Similar to the aspects desired in bookshelf offerings, the surrounds should be aesthetically pleasing to the prospective buyer due to the broad range of application of surrounds in everything from a dedicated listening room to a multipurpose family room. Frequency response for surrounds should also parallel the bookshelf offerings, possibly giving up a bit more in the lower midrange as it will not be as crucial for them to produce bass at less than ~65Hz, allowing either towers or a subwoofer to pick up where they leave off.
And that's all I've got to say about that...
-TD