I think you hit the nail on the head when you said:
"important information about projects could be lost by flattening the multi-level Trove hierarchy to a single collection of tags."
If we are going to lose important information why are we changing to it? And who decided which categories were most important? Platform is much more important to me than OS or Language. We are all geeks here, we understand how the trove worked, we like the specific. Tags work poorly as filters, you even had to pre-fill the license options because some people type "GPL" and others "GNU Public License"
"It is very hard to make something simple" You had it, why change it?