Some suggestions:
(I'm using Firefox)
1) On the trade page "My orders" at the bottom of the page are difficult to get to because of the secondary scrolling of the bid and ask orders in horizontal view. I'd recommend the area between the bids and asks with the button that flips sides, that this area not scroll. It is currently very difficult to find a place to put the mouse where I can scroll to the bottom of the page without scrolling through the entire bid/ask orderbook.
2) The precision on every market seems to be off by 1. BTS / OPENBTC for example shows 9 digits after the decimal instead of 8. so the current price of BTS in BTC looks like .000008123 instead of .00000812
3) When you first access the trade page. Clicking between My History and All History changes the width of the column. However once you click on My Markets or All Markets, it will no longer adjust the column width.
4) On the trade page the scroll bar extends over top of the top static row that contains the "market depth" / "Price chart" buttons. The scroll bar should be limited to the range of the window that actually scrolls. It also appears to extend long past the bottom of the page.
5) On the account overview page I think separating the column "Asset" into two columns Amount and Asset. This would make it easier to read and locate the asset you're looking for.
6) The Account Orders page columns are repeated for each market and do not line up vertically with each other.
7) The Trade page that just lists the markets fullscreen, lists the columns "Quote supply" and "Base supply" . I'd like to see the highest "Bid" and Lowest "Ask" listed as well. the base/quote supply is minimally useful. If they all fit great, but I think the Bid,Ask,Last is more useful.
8 ) Can we get a deeper history for the price chart. I'd like to see an entire years worth of data on the chart, or more, if possible. The default can be a small range so it loads quicker but having the option to see more would be useful.