I am an Apple evangelist, speaking the ways of the Macintosh, and have been since the days of Vista. I love OS X and the ease of use, the customizable interface, and the many benefits it has to offer. However, no OS is perfect, and despite what most Apple fanboys say, OS X is no exception.
The Dock. The Dock is a very nice feature and has been in OS X for quite some time. However, it lacks a few things, in terms of customization. Apple needs to put in "themes" for the dock. The glassy look is nice, but it gets old. And not everyone like the 3D look. Yes, you can download applications that modify the dock, but I strongly believe that this needs to be built in.
UI Themes. Like Windows, OS X needs themes for the overall interface. The gray look with colorful buttons is nice, but gets old. I would love to see Apple creative support for themes that are built in to OS X, such as an "Aero" look, similar to Windows. Again, you can download software to have a themes in OS X, but Apple needs to make their own, built in to the OS.
More options in Expose. Expose is great, especially in Snow Leopard. I still think it is unfinished, and could use more customizable options, and I also think Apple could implement Expose in other Apple software, such as iTunes, Safari, iPhoto and iMovie.
The return of support for third-party screen savers. When you upgraded to Snow Leopard, you probably noticed that all of your third-party screen savers no longer worked. It is unknown why Apple removed the support for this, but it would be nice to have it back.
These are just a few, but important, things Apple could improve OS X with. There are more, I'm sure. Comments welcome.