It's really just a lot of little things that make me despise using it for anything other than movies, or something that has my full attention.
For one, I'm using an illegal copy of Windows. Thanks to losing my original (Authentic and legal copy of XP) I have to use a cracked copy of Windows, as other SP2 CDs of XP Home don't work with my Product Key anymore. That means I can't update Windows Media Player from 7 to the most current version (I think it's 11). Thus I'm stuck with an older copy of it, as you have to have WGA installed to get WMP11.
You can't hide to the system tray. You either have to minimize to the taskbar, or enable WMP toolbar, which is just annoying, and just takes up more space on my taskbar. Something which is usually full of other programs already. I prefer powerful, but minimalistic programs to get what I want done.
Media Monkey lets you queue stuff you're searching through as either the next song or just add it to the end of your current playlist.
It's a lot of little abstract things that I don't feel WMP does as efficiently, or I haven't learned the equivalent of on WMP, nor am I interested in figuring out at this point.