Erik has one, and I’ve been meaning to make one as well, so here it is (with significant overlap with his):

1. Improved performance. Sure, doubling my RAM to 2 GBs will help, but there’s really no good reason for Aperture to be this slow on a 2×2 GHz MacBook Pro. I’m not even working with RAW images yet, only JPEGs.
2. RAW import API. Perhaps there already is one. Either way, since Apple doesn’t have the resources to / cannot be bothered to add RAW support to a more reasonable range of cameras, manufacturers should be able – and strongly encouraged – to do so themselves. I’d probably prefer a plug-in of theirs (with a standardized Apple-mandated user interface) over an entire mediocre bundled app by far, even if it means slightly more limited control over the output.
3. Adjustments API. Aperture does come with a range of adjustments that suits the requirements of everyday photo image editing. In some cases, though, third-party adjustments would help. Like Erik mentions, an integrated Noise Ninja (or similar) would be far better than the quite limited built-in noise controls, and would save you the painful round-tripping.
4. Local editing. That is, having adjustments not affect the entire images, but select parts. Nikon’s Capture NX is particularly strong in this area; intelligent selections (rather than manually created image masks) certainly help the workflow a lot. All adjustments should be enhanced to allow for this.
5. HDRI. Like you couldn’t see this coming? And I don’t just mean a nifty UI for layering multiple photos with different exposure settings (but otherwise virtually identical) together; tone mapping should be included for the export as well.

That’s about it for the major things. All five things would help my workflow tremendously, and especially prevent almost all round-tripping (in the words of Jerry Espenson: “yuck, yuck, yuck”!) I currently have to do.

Now, as some minor aspects:

1. Customizable full-screen toolbar. What’s up with that, anyway? I can customize the normal toolbar. Perhaps, the two should just be kept sync.
2. A metadata HUD for full-screen mode. Unless I’m missing something, I have to leave full-screen mode and use the regular sidebar palette instead.
3. Much better keyboard-based toggling between metadata input and UI keyboard shortcuts. I keep accidentally typing a ` in the keyword or version name fields, intending to invoke the loupe, not realizing the text field still has focus. Conversely, I can’t think of a simple way to activate those text fields via the keyboard when I want to. Mouse navigation is needlessly inefficient in this context.
4. Much more background processing, such as of exports. Why have a modal dialog when exporting? I’d much rather continue to work on other images in the meantime.

Oh, and Front Row support. Just kidding. Mostly.