It would be nice if the login lasted longer. It currently lasts less than a day. It also will say you're logged in, but when you click to add something to your watchlist, will take you to the login page again, after which it doesn't redirect you back to where you were.
These two messages appear at the top of every page no matter how many times you close them. They don't seem to contain particularly critical information either.
This is just a usability thing, but it would be nice if you could choose which information to show about an item. I don't use the "Sell listings mv 1 day avg" and actually don't quite uderstand what they are because I'm dumb. I only use "Sell listings raw data", so every time I look at an item I have to click off the 7 other graph lines.