UI improvements.
Or possibly the ability to skin the site. Although that is a much bigger project, and UI improvements are likely needed before something like that could likely be done.
As a new user, I've noticed a lot of little annoying things about the site UI. For one, the upvote button is way too small. Perhaps that was an attempt to limit the number of people pushing it? I have to move my mouse very carefully to hover over it, and then press it.
The resteem button is also pretty small, but I use that far less.
I also haven't figured out how to reverse a resteem yet. Sometimes after resteeming something you decide it wasn't worth it, as it clutters up your feed.
I also think certain pages should have their own dedicated button on the top, like your feed and replies. You shouldn't have to click into the drop down menu to access these.
On a side note, as a new user, I had no problem with the sign up. It was quick and easy. Edit: Well, the approval process was slow, but I assumed that was due to manual approval. If not, and that's what you meant, then I understand.