As jasongullickson said, a reasonable price.

But I think the question is really strange. It's like asking "What is the Porsche sedan lacking that the Volvo sedan provides?"

They're for different people. Photoshop in my mind is for professionals who use a lot of different tools to master their craft. While Pixelmator seems much more focused on photo-editing and some quick work.

Pixelmator makes design and (more advanced than iPhoto-) photo-editing more easily available to non-professionals.

If you really use everything Photoshop has to offer, I don't think the price is unreasonable. But few people do.

A reasonable price.

No activation/licensing bullshit, feels at home on my Mac, does not try to take over the machine.

It's very small, it's very fast, and it's very cheap. Feature-wise it doesn't offer anything that Photoshop doesn't already have.