This is more of a "research problem" wrt UI's, but I think the tagging system needs a better UI overall.
Assuming that tags are supposed to categorize content (akin to sticking labels on things), a good tag system should make things easy to find.
But what you have is a kind of soup of things with labels on them.
You can search through the soup pretty easily. Tags as they are, are kind of browsable through the search box UI. You've made it work, kinda.
But I don't think browsing on SE sites, especially those that house multiple topics, is as pleasant or clear as it could be. What would be clearer? I'm not exactly sure, but that's why I said this was a bit of a "research" problem.
The newest proposed regarding music.se is to combine all music interest into a single site, instead tons of smaller, less significant mini-sites.
To have violins, guitars, piano, music-theory and song-writing all on one site, you have to be able to organize things quite well.
I think the tagging system is sufficient. Even with the 5 tag limit on each post, tagging is enough to cleanly divide the site, but its the front end UI to the tagging system that needs work.
Tags replace folders, and tags are arguably better because they don't force you to repeat structure. For example, if using "folders" on SO:
Folders are bad example 1:
C++ operators variables
You'd also have
C# operators variables
Or you could do
Folders are bad example 2:
operators C++ C# variables C++ C#
with repeated structure. With tags you essentially get "virtual folders"
Tags are good example 1:
C++ & operators C++ & variables C# & operators C# & variables
Can't do that with folders, and the beauty is, you can VIEW tags as folders.
So perhaps a user-configurable navigation structure that presents a tree view.
You select your "root" tags, for example,
guitar (or these can be hard-set by the site manually OR tags can become root tags when there is excess of 10,000 or so posts in them)
piano technique fingering guitar phrasing strings effects
So, with piano as one of your/the site's "root tags", the site takes on an artificial folder structure on top of the tag system.
Each post must contain a "root" tag, which roughly defines the site's scope (kind of like how MSO has "discussion" "support" etc). This helps you define the scope of the site, and in addition it gives you the opportunity to create a hierachical browse mode, instead of tag-soup mode.
So one possible UI for a front page that has root tags might be
song are defined as root tags, and the topics shown below are popular tags that also have that root tag.)