There's some buzz about Leopard (OS 10.5) coming out and how it will compare to Vista when it finally rears its ugly, but shiny head.
Frankly, I don't think it's a fair comparison. (No, I'm not even remotely concerned that Vista will be better than OS X. I just want to make sure the comparison is fair so when Leopard blows it out of the water it will be all the more spectacular.) Leopard is and incremental upgrade, one of many made over the past six years or so. Vista is the first upgrade made in the same time period. So if you look at the features Vista offers compared to its predecessor versus the features Leopard offers over Tiger, you're more likely to be impressed.
Apple has been allowing their customers to enjoy new, innovative features over the last six years thanks to consistent updates while Microsoft has apparently been too stunned at their progress to do anything for their customers but issue security patches.
In short, if people are going to compare Leopard to a Microsoft product (which they inevitably will) it should be against XP, not Vista.