A successor to nvALT, you say?
The biggest difference is that it works with multiple folders and sub-folders. You pick a folder, it indexes it, and you can use it just like nvALT. But then you can open another folder, or create a new one and start editing. It allows you to create folders anywhere, maybe one on Dropbox or iCloud Drive that’s shared, one on an encrypted disk that’s private, one for work, one for home, one for every writing project. You’re not limited to tags (though you can search by and sync with macOS tags within the app), and you can sort your notes into subfolders as well.
nvALT is one of those tools that’s not as polished on the UI front as, say, Bear or Ulysses — but it’s so fast that I still use it for note-taking (especially when hooked up to another editor like Byword for writing longer notes). Speed is a big part of UX.