I'll admit -- I've been reticent to migrate my MacBook Pro to Leopard. The Tiger installation on that system has been stable and reliable, and I didn't want to visit any unnecessary demons on it if I could avoid it. However, I just received my MacBook Air that came with Leopard. This has given me a few more reasons not to upgrade, unfortunately. The biggest issue is X11.
Tiger's version of X11 was cranky right out of the box, with X11 windows not gaining focus when Command-Tabbing to it, and middle clicks to paste the clipboard buffer don't work, and a few other inconsistencies. After a few months, an update was released that fixed these problems. It seems that the Leopard version of X11 again suffers from a nearly identical set of problems. The version included with Leopard is v2.0, where the version running on my Tiger MacBook Pro is v1.1.3 -- and it seems that I may need to downgrade to Tiger's version to resolve these issues, though obviously, I'd rather not do that.
The vast majority of Leopard users will never know about, nor need X11, but there's a large contingent of deep geeks that use Mac OS X and X11 constantly -- and this is the audience that greatly influences purchasing decisions made by others. It's certainly in Apple's best interests to fix these problems, especially since we've been down this road before. So please, Apple, please fix Leopard's X11. Pretty please? With sugar on top?