Agree, eSATA is the best way to go for external HDD.
Thunderbolt's first hurdle is cost, if too expensive the vast majority of users won't pay for it given a choice, the second is cost vs productivity increase which is subjective.
For my personal laptop/desktop the speed increase won't justify the cost at this time.
For a small percentage of business laptops/desktops it may be viable.
It does have potential for improving SSD performance.
If the cost is reduced and it becomes a standard, bringing the cost down further, it has a shot.
As far as a technological performance increase it's a great idea.