
- It's not centered
- The add to cart button is too big and plain (IMO)
- All elements are stacked horizontally so each item takes a lot of room, with wasted space on the side.
- Doesn't line up nicely with other products having more information or ratings..
You could fit a lot of information in a small place of 1.4.x:

While the same space shows only 2 products in 1.5.x. even if you shrink the images to the same dimensions. It still stacks it all.

Suffice to say I'm not a huge fan of the default 1.5.x theme, especially not when compared to 1.4.x which I think still has one of the best default cart themes of any cart.
I think the html is fine, but the css needs work.. again... in my opinion.
If others agree, I'd like to hear some css-only fixes to improve the default theme.