As far as I can tell, the difference is this:

UAC is per-process and monotonic. Once elevated, the entire process stays elevated.

The new model is per-operation. Even if the same process has been allowed to elevate before, it must ask to do it again. I don't know how granular this is, and whether there's a grace period like sudo.

However, the biggest problem with UAC was that it was considered too noisy for the end user, leading to people just blindly accepting every dialog and Microsoft turning down the default level to the much less secure "don't always prompt". I don't know how this new model will address that problem; naively, it seems to be worse on this front.