Inherent Understanding and Adaptability 

In this interview with Bryan Fuller (Already cited on Sylar's page), he says that Sylar's power is an "inherent understing and adaptability", and he  was able to use his powers in the cell to kill Eden because he was "playing possum". (The Sylar part of the interview starts at about the 42 minute mark). Given that description, I think that his original ability is comprised of two seperate comic-book superpowers:

Superhuman Intelligence - Sylar has inherent understanding of how things work.
Reactive adaption/Evolution - Sylar can adapt his own body to various conditions. It's on a much more subtle scale then in comic books, granted, but it's pretty close.

Thoughts?