Does Steam prevent changes to game files? There are a lot of games that benefit from tweaking or third-party patches for performance improvements or to improve compatibility with newer hardware features. Does Steam lock down game files in such a way that make INI tweaks, DLL replacers, memory injection, executable wrappers, third-party patches, or other modding methods hard or impossible?
These sorts of hacks tend to be more common and more important for older games. It does seem like some tuning is possible, but that example is for a DOSBox-based emulated game, and I just don't know how much Steam will let you do to an old Windows game.