It's mostly the colors - they don't contrast enough! The border and background colors are very close in every regard, but it's mostly their very low saturation. Not the best for HUD/UI elements. I did a quick 'n' sloppy edit for you:
If you're dead-set on that palette, though... it would be hard, but not impossible. You'll just have to make use of the brighter colors for anything you want to be easily distinguished (like the borders).
On to less general things - the font you're using has aliasing, which conflicts quite a bit with the style a low-color palette brings. It's not a huge deal, but it might help aesthetic if a non-aliased font were used instead. Another thing to consider is readability - if you want the player to read through the list and see a familiar image, consider making your item images take up more of the panel. On the contrary, if the main thing for them to look out for is a certain stat, make the numbers for the stat prominent. Right now there's a lot of empty space in panels, which you'd preferably want to reduce to 0 - the panels should have just enough space on them to show everything important (and not feel cluttered). I'd also recommend eventually using logos to represent things like Damage and Speed, since those are fairly easy to understand in logo form and help with readability.
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