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Seriously! Look at the phrase. LOOK AT IT!!!!
The implication being that at one time a particular game was "intelligent" but has undergone some unholy voodoo and been turned into a version of Chutes and Ladders or, even worse, Ludo. This term is normally used in reference to a multiplatform game that was made for consoles and PC at the same time.
What's so confusing is when the term is applied to a brand new franchise that plays exactly the same on all platforms. How can you "dumb down" something that didn't even exsist previously? You can claim that one control type is less efficient than another, but that isn't the same as watering down the core mechanics.
Let's take Command & Conquer 3 as an example. Both the PC and console versions of the game use the exact same maps, the exact same factions, the exact same unit caps, and the exact same units. What exactly was "dumbed down?" Same thing with Morrowind. Both the PC and XBox versions had the same maps, the same kind of first/third person minimal interface, the quests, the items, the same NPCs... If it's the exact same game at it's core, how can it be "dumbed down?"
You might be able to say this about a game that started out on the PC and was poorly ported to a console, like CiV 2 being placed on the PSX near the end of it's life cycle, but If it was developed for both the PC and consoles or it was a one-for-one port....
And while we're on the subject, was Assassin's Creed "smarted up" by being ported to the PC?
I'm calling bullshit on this term and it's use.