now at sonickind - Things tumblr should change/fix:

Just my opinions.

Tags - Nothing ever shows up consistently. Maybe have a very small gray arrow at the ends of tags on posts so you can scroll through all the tags on them by just clicking on the arrow.
Theme Garden - They are all on one page which makes it take years to load, there are only a few sorting options, they could add tags/a tag cloud/a search to make finding a theme you want easier.
Directory - Other than the sticker fiasco currently bothering everyone, I would say the same as the theme garden. Rather than having one page and clicking a “see more” over and over they should just have multiple pages. Also a search option would be nice.
Following/Followers list - There is a clear way to tell who, out of your followers, you are following back. All you have to do is see if there is a “+Follow” button next to your follower’s username. But there isn’t an easy way to tell who out of the people you are following, is or isn’t following you back… other than copying and pasting names back and forth which is tedious and annoying. Also the options to sort alphabetically would be nice.
Replies - This one is obvious.
Messages - It could just be a bug, but at the moment they don’t disappear when you answer them. It would be nice to see that fixed.