I think this design misses the mark. In many ways it feels like a store when it should feel like a conversation. Some thoughts:

1) It doesn't feel like there's been enough thought about the various segments of people who will use this. The design leads with Popular but does popularity really matter? Same for Trending and Beta. The front page should be an answer to a question which is "I need a plugin that does X, what are my choices?" No one really is thinking "You know, I want to install the most popular plugins, regardless of what they do" or "Hey, I want to install some trending plugins on my site!".

So, it's important to understand WHY people come to the site. What they're looking for. What problem do they have and how can the site direct them to a good set of solutions?

2) To me, this means the design should be that of a conversation and the search interface should basically help the visitor ask a question. I'd start by doing this almost literally, i.e. with a form that says something like "I'm looking for a plugin that does _____________" where ___________ is a search box. As they type, show matching categories and tags.

Alternatively, show the top level categories (and there really shouldn't be THAT many - maybe 15-20?) and let the visitor choose what they want. Use faceted search to refine that ("Over X Stars", Updated in the last N (week/month/3months/ 12 months) etc.

The design as shown in the mockup doesn't come close to doing any of this. It's designed for the plugin authors and not for the people who are using Wordpress, find that they need a new feature and are hoping to find a plugin to use in order to implement that feature.