I'm aware of at least two important differences between SATA and eSATA. eSATA is designed to use longer cables (makes sense for the intended use), but I don't remember the two length limits. And eSATA ALWAYS includes support for Hot Swapping, which may or may not be included in a SATA controller. So using a connection adapter to allow an eSATA enclosure to hook to an internal SATA controller will work for many things, but just might not work for a couple of funstions like hot swapping.
I think there are a couple of other differences, too. Like, the eSATA connectores have a better way to hold onto the cable plug for a secure connection.
As far as I know, all of this is in the eSATA controller. You do NOT need a special drive in the external enclosure. BUt of course the enclosure will have a special eSATA connector.
Much of the discussion reminds me of actual real-world testing results I saw last year, done by copying a full large HDD to an external enclosure, but using various interfaces. I believe USB2 was slowest, IEEE 1394a (Firewire 400) a bit faster because of less bus management overhead, eSATA certainly faster than those two, and IEEE 1394b (Firewire 800) was best at about twice as fast as USB2.