Pros:
-3D engine makes the world even more immersive
-Graphics are so good the quality is as good as the pre-rendered slides of the original--no matter where you are or where you look.
-freelook ability makes some gameplay less frustrating (finding hidden doors, for example-- you don't have to click one frame, turn, turn back, turn other way, turn back, click forward again, repeat...God i hated that)
-with 'advanced mappings' enabled, character control is the standard and familiar WASD
-A new age not included in the original (just a short bonus as a reward for completing the game)
Cons:
-I could not get it to run correctly (at all) on Win7 x64. Looking at multiple forums, I'm not alone. Worked fine on XP.
-If you have played Myst before, I'm not sure the replay value is worth buying the game again...