OK, as long as we're not trying to define other people's selection processes, here's mine:
1: Focal length. It feels funny listing this first but covering multiple focal lengths is the point.
2: Sharpness / performance in general. I suppose prime/zoom can be stuffed into this category also - as in I haven't found a zoom yet that was a long-term keeper. While I kidded the original poster about this thread and the one true criteria, my lenses do have "4.something" ratings on photodo. Photodo results alone are a tremendous simplification of lens parameters but I think they're a useful tool. When I first discovered Photodo, I learned that the lens I prefer above all others had a 4.6 rating so I figured that Photodo and I were a fairly good match.
3: Aperture. This is really hard to separate from the others. For example, while I may have been thinking 100mm, the 85 f1.2 won for speed and performance. Generally I look for 1.2 to 1.4 for wides, through mid telephotos. It gets a little more ambiguous at the high end. Also, I will sometimes give up SOME performance for speed so it's hard to keep #2 and #3 in strict order. For example, for a "standard lens" for my 10D, I looked at the 35mm f2 which had a Photodo 3.9 rating and the 35mm f1.4 which had a 4.0. Well that was a no-brainer but if those performance numbers had been reversed I would have still gone with the f1.4. On the other hand in the 50mm, the 1.4 seems like the "sweet spot" vs. the 1.0. (To me.)
4: Feel. It has nothing to do with the picture but I like a tool that really feels good when I use it. Sadly, I think the EF lenses don't quite measure up to FD lenses in this regard but I've finally given up on Canon producing an FD digital body 
Other factors are much more mutable/subjective/changeable. For example weight can be a very minor plus up to 200mm or so but can start to become a major minus after that.
Price does come in but over the long run (I've owned Canon SLRs for about 30 years now) I eventually end up with the lens I really want so there's always the question of how long a stop-gap lens will be used.
IS sounds nice but so far, all of my long telephotos are still in my FD line. As I'm converting those to EF I'll need to evaluate that.
Filter size doesn't matter as much to me as it once did as I'm using them less. I only recently gave a way a bunch of filters that I used exclusively with Infrared Ektachrome. Yep probably don't need those for the 10D...
That's just my way. Your mileage probably will vary.
) salary does it cost.
