I think iTunes has all of those things you want nowadays, actually. At least for me on my Mac. It seems to kinda suck on other platforms because it has to drag a whole lot of the Quicktime infrastructure it relies on for playing music along with it, and becomes kind of big and unwieldy.
I get system notifications of playing tracks if I want 'em.
It's got plugins. I don't know how extensive they are, mostly I just have a handful of visualizers, and I think I've got a couple music format plugins somewhere in there too. (I've spent like $100 on various visualizers over the years, right now I'm really liking Aeon.)
I'm not sure what "excellent media library" or "the ability to play straight from the library" entails. I can double-click on a track in the iTunes window and it plays it, it manages all my folders and tags for me and makes it easy to dig out the actual files when I want to, I can sort in all kinds of ways.
Does Winamp do "smart playlists"? Because that's iTunes' killer feature, IMHO. It spends the vast majority of its time in a playlist of "stuff I haven't played in the last 5 weeks, or skipped in the last 10". Keeps my vast collection constantly rotating.