How does Serious Sam HD: The First Encounter differ from the original game?

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?

From http://en.wikipedia.org/wiki/Serious_Sam#Serious_Sam_HD:_The_First_Encounter

The game is a remake of Serious Sam: The First Encounter with improved textures and character models. It retails for 1200 Microsoft Points or US$15 (also $15 for the PC version) and supports 16-player online co-op (4-player in the XBLA version). This remake features Achievements.[5][6] Deathmatch and dedicated server support have recently been added.

I actually purchased both HD versions as I did not own the previous ones but played them at friends.

Kr,

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:

• On the Sand Canyon level, the game engine changes made it a lot easier to just run up to two of the secrets that used to require some tricky jumping. So much so that if you didn't know the game's history, you'd probably wonder why they counted as "secret" since they're in plain sight.

• There is one extra secret on the Dunes level.

• On the last level, The Great Pyramid, the original game had no secrets. The HD version has 3 secrets on this level.