I own the original Serious Sam: The First Encounter and I absolutely love it. What has changed in the new HD version? Is it worth a new purchase?
I actually purchased both HD versions as I did not own the previous ones but played them at friends.
For the most part it's the same game with updated graphics like the other answer says, but there are actually some distinct differences.
The original game had some anti-gravity effects on a few spots in the levels that were not in the HD version.
There are also minor secrets differences between the two games. I wrote a secrets FAQ for Serious Sam when it originally came out, and when updating it for Serious Sam HD I noticed some changes which I'll list below: