For me the killer feature of foobar are the global hotkeys. I browse the files in my fav. file manager and do all the previewing/seeking with global hotkeys. My hands never have to leave the keyboard. I hardly ever look at the player - all I see is the tray icon.
Apart from that it's mostly little things. The whole concept of the player is very logical, consequent and pragmatic; no unneccessary bloat. Form follows function.

Native support of ape2 tags on many file types is very nice. Broad support in the community i.e. plugins for "obscure" file types (ok, Winamp is strong there too). Being an mpc user myself owning an mp3 portable, I enjoy the easy conversion from one format to the other preserving tags. The integrated ABX tool motivates quick tests (I don't have to locate the files on the hdd and decode them, load them into a standalone abx utility..). The flexibility of the playlist display is impressive and it works globally for all file types.
Those are some of the reasons I prefer foobar over Winamp (and the few other players I tried. I've never tried any of those huge all-in-one apps like MusicMatch or MJ). I suppose others here prefer it for similar reasons. It's not all just blind following. I don't care much for version numbers, but I do stick to the stable releases. Foobar already offers a large set of benefits over the other players I tried. ymmv.