Before I start giving my thoughts on the game, let me tell you that Sonic 2 HD has a few new features that make this version different from the original. First, you will notice a change in the level design - there are now new zones that you can explore in order to complete all levels. Second, there are now many new customizations to Sonic's moves and skills, which give him a whole new feel and appearance.
These two features built-in tools, together with the brand new "Chao" system allow you to create your very own custom character in the game. The level of detail added to the graphics card is truly staggering, and the streamlined development engine allows the developers to create many new features in this version. In my opinion, the improvements and features built-in to the Sonic 2 HD engine are very good. Compared to Sonic CD, the graphics have a higher resolution and are more detailed. Also, the game's overall stability is better than its older predecessor, and the user interface is snappy and smooth.