The trackpads on the newer Macs (2008 Unibody models to current) are just much much better than any other computer trackpad. And they are better in every criteria.
Smoother Texture
Wear Better
More Accurate
Accurate gesture recognition
Larger Surface Area
Some others have mentioned about the glass, this is a fundamental reason for a lot of its special qualities. The glass doesn't wear as fast as plastic does so you don't end up with a trackpad that looks like this:
admittedly this is an extreme example most trackpads that get worn end up like this:
this is due to the oils secreted by your skin working its magic on the trackpads wearing the plastic down like a high quality abrasive. It makes them shiny and in some cases this shininess can make the trackpad less easy to use, it causes friction.
But it's not just the feel, it looks awful and seriously hurts the resale value of your notebook. I have yet to meet a notebook with a plastic trackpad that didn't suffer from this problem and that includes Apples own notebooks from years ago (iBook, PowerBook, early MacBook Pro's before Unibody).
The glass however holds up. I used the same 2009 Unibody notebook almost every day for 4 years and 10 months and the trackpad looks fine, no marks, no blemishes. Every inch of it looks the same unlike the plastic trackpads that visibly wear in the centre where your fingers spend most of their time.
I've even seen plastic trackpads on Windows PC's start to gain the shiny blemish within
2 months of normal use. Not to the severity I posted above of course but enough that it is noticeable and the surface texture is already starting to feel deteriorated.
But honestly there's nothing intricately unique with what Apple has done. They haven't cracked a secret formula or bottled unicorn tears. They saw a problem and they solved it. No doubt the glass costs more than plastic and that is what drives all these second rate OEM's to use plastic with poor results.