"Dumbed down" is a catch phrase used to describe changes to interfaces, technical specs and gameplay elements. It rarely has anything to do with anyone being dumb(other than occasionally the developer/publisher), so I laugh when people take offense to the term. In the case of PC to console changes. the most common "dumbed down" complaints boil down to the following:
1) An interface is optimized for consoles, thus making UI navigation pointlessly tedious for PC users who are accustomed to being able to assign actions, menus, etc to hotkeys of their choosing. Especially annoying due to the relative ease with which this could be optimized for each platform differently.
2) Graphics and/or AI are designed for the lowest spec device, which is almost always a console. This makes games look somewhat bad in comparison games that are designed with PCs in mind, though of course good art direction trumps technical advancement.
3) Gameplay elements are changed for reasons related to the console platform, either real or perceived by the developer/publisher. Auto-aim is an example of a change due to real limitations, as controller sticks are not as precise as a mouse. Making challenges easier, reducing data available to the player, and removing non-core gameplay elements are changes made because the developer feels that it will appeal to a larger audience, rightly or not. Since that larger audience is perceived to be on consoles, the blame is often placed on that audience, again, rightly or not.