During my review of NetFlix's "Watch Now", I mentioned that I had to do a rebuild of the Windows Media Player library -- it got damaged because I had the player open in its normal instance when I launched a "Watch Now" window, and I tried to close it. I passed on a warning about that, so that other people don't make the same mistake -- and I hope it can be fixed in future iterations of either WMP or the "Watch Now" client.
The results of the rebuild process itself, though, was pretty interesting. I have a lot of music in my library -- some 15,000 songs, or about 100GB of music -- and I found to my surprise that library actions in WMP took place much more quickly after the rebuild. I suspect the library database is prone to internal fragmentation, and that forcing a rebuild from the ground up helped resolve that.
Another interesting tip: When you close WMP and you're in the middle of a playlist, switch to the Now Playing tab first before closing the program -- that is, if you're not already there. This seems to more consistently allow the program to come back up not only with the same playlist, but at the same song you were playing before the shutdown. I noticed that WMP did this before, but it wasn't until after I conducted a bit of experimentation that I determined the exact circumstances for how the playlist is preserved.
You ought to take a look at JRiver Media Center for music management. It's built for large music libraries and has a lot of customization and organizational options that WMP doesn't. JRMC has an active user community and the company releases updates at least weekly. It also synchs with iPods. Standard disclaimer - I'm not being paid by the company - just a happy customer.