Well, since I just bought a Skopar 35/2.5 Classic today, I can say that I bought it for: its price, its focal length, its size, its sharpness, its color rendering, and its reputation. For not much more money I could have gotten the pancake M version new instead, but I really like the focus tab on my Skopar 21/4, so it seems that that is also something I look for in a lens with all else being equal.
All that said, I can think of other lenses I have that meet almost none of those characteristics that I still love anyway. My old Elmar 90/4 comes to mind. It's a little big for my tastes (even though it's small for a telephoto), no focusing tab, isn't sharp wide open (or even stopped all the way down, for that matter), and it could use a CLA as the barrel is stiff and it can be difficult to focus. It has dust and cleaning marks. The lens cap always falls off. Yet it makes wonderful images, especially portraits, and for what I paid for it--$75--it is an absolute steal, considering the use I've gotten out of it.