Vinny, You are correct that a plug cannot know what neds to be corrected. EZMix and similar packages make some asusmptions. For example, a common trick is to boost a kick drum at around 60-80 cycles, cut it around 200-400, and boost it around 3000. So, one kick drum preset may have settings like that and be called "Std Kick" or something. EZMix has a variety of kick presets for different styles of music. They all assume that the kick you are processing is fairly normal but you will have to try the presets to see what you like.
The same goes for vocals, guitars, etc. EZMix combines a number of effects into each preset, so a guitar preset may consist of EQ, compression, delay, and distoration, or some other combination.
For mastering, you will either have to know something about what you are doing, or go through the presets on a trial and error basis. Mastering does not seem to be EZMix's main focus or biggest strength, at least in my opinion. It seems to be better at processing individual tracks or submixes. That said, I bought an add-on Mastering pack for EZmix 2 and it has a couple of presets I like.