on a small scale, the sleeve design certainly has an influence on sales i think -- at least i'm much more inclined to actually buy a CD (or any other product) when i like the design. but on a larger scale, if you look at how music is being marketed and how little this actually has to do with the music, i don't think that designers should care. i definitely don't.
i'm not so familiar with CD shopping, to be honest -- sure, at large chains you can't open the package, but then you don't need to go there. that's the alternative, i would say: buy at smaller stores where the owner will let you have a look ... maybe he'll even have a clue about music. (but then of course i live in berlin where there's plenty of small stores everywhere -- not sure what i would do if i'd live somewhere in the woods)
depends on the job -- for larger releases it's certainly important to comply with the distributor's standards (dimensions, info on the spine, usage of standard jewel-/slimcases, etc), but the smaller indie labels mostly don't care. what's more important for them is manufacturing costs; it can make a huge difference whether you print cmyk or spot colours, use this material or that, etc ... being aware of these things is part of the designer's responsibility i would say. at least that's how i see it – i can't really do a design without taking the material into acccount.
this also depends on the job ... i don't have a general rule. sometimes i just listen to the music, sometimes it's more a general concept -- it really depends on the project. for a pop songs album i wouldn't look for a "concept" at all, but in non-mainstream electronic music things can sometimes get very interesting. i also have experiments in the drawer waiting to be printed ... so the "inspiration", or the general ideas, are not always strictly "musical" ones.
not sure about the superlative, but one thing i really enjoy is that when you do a sleeve for an album, the task is very clear and concrete -- you listen to the music and then you know what you're working for. as opposed to, say, the amount of hot air involved in creating a new "image" for a "brand" which wants to be more "innovative" or something ... sure, sometimes (very rarely) musicians also just ask for something that "looks cool", but that's really an exception.
some CD covers are desirable and collectable and some are not, just as match boxes, flyers, vinyl covers, postcards and other things are sometimes nice and sometimes ugly. there's only so much a designer can do – the situation where the designer is in the position to change the look of things fundamentally is a rare exception i would say, if you look at "products" on the "market" in general (not just record sleeves, but the abovementioned postcards etc.). i guess that most of the stuff that's being sold is "designed" by marketing people, not designers.
as for CD vs. vinyl covers, i don't see a fundamental difference between the two -- there's just as many well-designed CDs around as there's LPs. or probably even more of them, since a lot of albums are released on CD only so there's simply more CD covers to start with.
as i said above – on a small scale (in the stores) it surely does, but on a larger scale it's the promo budget that makes the sales, not the sleeve or the music.