There are 3 things I've noticed so far that are different:
When creating sub-tags the tag structure collapses and you need to re-navigate to the sub-tag that was just created. This also happens when I drag+drop an item onto a sub-tag. I'm constantly re-expanding tags / sub-tags. I know that's a minor complaint, but the amount of clicking and scrolling I find myself doing in general now is pretty annoying.
Like apollo13 said, duplicate tags aren't allowed. We use the folder structure to organize by customer, then service, so we have a lot of duplicate names. For example:
In Enpass 5 I could navigate to a folder and any items I created after that were automatically put into that folder. Now I need to either enter the tag name by hand, which is extremely difficult given the number (and length) of tags we have. Alternatively, I can search for an item after creating it and drag+drop it into a sub-tag. To use Office 365 as an example, Enpass 5 was really good. I'd navigate to a customer's folder, create an "Office 365" sub-folder, and add passwords as I created users.
When I saw the tags in the beta I assumed they were going to work almost identical to folders, just with a different name. I regret that I didn't test it more thoroughly. All of our phones have auto-updated to version 6, so I'm going to end up updating to v6 on the desktop and will have to live with all of those annoyances for now.
#2 seems like an input bug. When I try to create a sub-tag called (ex:) "Office 365", it tell's me the tag exists. However, when I look at the tags listed in an item, it seems like the tag is actually "Customer 1:Office 365" or "Customer 2:Office 365", so it's not really a duplicate, is it? I can also work around the limitation by typing a new tag manually. For example, "Customer 3:Office 365" is allowed.