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Aside from Aperture 2's new Recovery and Black Point tools, the tool that's impressed me the most has been the Retouch Brush. I had a love/hate relationship with the "Spot and Patch" tool in Aperture 1.0, if by "love" you mean hate and by "hate" you mean hate. The Retouch Brush, on the other hand, is purely loved.

If you mouse over the image below, you'll see a very quick before/after. I scribbled over the offending area using the defaults for the Retouch Brush. I should have probably fine-tuned it, but I didn't bother, and the results are pretty great, even for a fairly large item which blocks a good bit of the sky.

Here you can see a 100% magnification of 90% of the area occupied by the tree on the left. A closeup reveals that a little smearing and smudging still exist - the sky goes a bit darker than it should, particularly where the shot had mostly tree limbs before - but in the full-size shot these parts simply blend into the cloud.

I'm very pleased with the Retouch Brush tool.