Well, 2E, 1E, and oD&D had their fair share of lame-tastic or appeal-deficient monsters (the bhaergala and tirapheg leap to mind), and your point that we'll never all agree is well taken. For example, I think that, aesthetically speaking, assymmetrical bodies and sonic based attacks are lame, but someone on the 3E design team likes 'em because that's what's in there.
I still think that the MM creature selection was done to serve gamist design principles at the expense of D&D atmosphere, storytelling and aesthetics, and the result of that is perhaps why I think that it's something of a disappointing book in those respects.
It is admirable that the designers were attempting to support levels beyond 1-10 with the monster selection, and make bards feel useful occasionally to boot, but I think that this was done at the expense of other considerations which make a monsters appealing - and, more importantly, D&D as a game appealing.
I hope they seriously consider an approach such as going through old Dungeon magazines and modules and counting which creatures end up in the most adventures when determining what should appear in the 4E MM. To state my assumptions, those monsters which appear the most in published adventures are probably the most useful for constructing adventures (which are the heart of D&D) or appealed to the module authors enough that they were included, and therefore have the most right to make it into the core book.
Who knows - maybe the 3E designers did that, but it sure doesn't look like it to me...