I really think that from the NES generation forward, the past five generations (including this one), that developers, publishers, manufacturers, and marketers, have focused on the same group of people as a moving target, re-tailoring their focus as that group aged

It makes sense if you break it down thus

3rd Generation (NES, Master System, 7800): Families bought in to the NES and it's ilk largely as toys for their children. Toys appropriate for 5-9 year old children, who would compose the target consumers of the generation. Games like Super Mario Bros, Alex Kidd, Zelda, Megaman. The headlining games of that generation were known for their simplicity and accessibility, holding the same learning curve of "easy to learn, difficult to master" that set the bar for generations to come, mostly to appeal to children. Bright colors, easy to follow stories, but something that you could play for hours on end

4th Generation (SNES, Genesis, TG16, Neo-Geo): These gamers who bought in between the ages of 5 and 9 between 1985 and 1989, attracted by the family appeal of the NES, have now grown to 10-14. What do pre-teens and early teens want? Something that's edgy. Something that isn't balls-to-the-wall brutal or violent, but something with attitude. Something that validates their "maturity" without being too mature. Something their parents will let them buy, but won't make them social outcasts. Something like Street Fighter, Mortal Kombat, Sonic the Hedgehog, Streets of Rage. Of course these have varying levels of violence, but they all have "attitude" which was all the rage in the early 90's, and was perfectly taiolored to pubescents. SEGA particularly capitalized on this with the rise of the Genesis

5th Generation (PS1, N64, Saturn, 3DO, Jaguar): By now, you're looking at a crop of teenagers, 15-19. Now you're looking at the beginnings of a different kind of maturity, and you saw the craving for realism emerge. The new maturity meant that people wanted "serious games" not the kind of cartoony, crazy silliness that reigned supreme before. Thus you saw the great success of stuff like Gran Turismo, Ridge Racer, Metal Gear Solid, Goldeneye 007, Perfect Dark, etc. Serious games to validate an audience reaching maturity

6th generation (PS2, Xbox, GC, DC): Now this generation has moved on to college (20-24), and have become separated from their old friends, which coincides neatly with the rising demand for online play. Halo 2, Phantasy Star Online, SOCOM, As the NES generation entered it's diaspora, they stayed in touch through network play, and continued refining the same tastes for serious, mature titles

7th generation (Wii, 360, PS3): Out of college and in the real world (25-29), this generation now has disposable income of it's own, and it coincides with a jump in console launch prices. With more money, you saw a rise in "epic" games, more people pushing big-budget projects to compete for the disposable income dollar. A new level of production values with stuff like GTAIV, Metal Gear Solid 4, Gears of War. Of course, Wii bucked all these trends, and will perhaps "reset" the whole system. But considering that Wii reaches out to all age groups, and not merely children, it may simply break the chain altogether, but the pattern is definitely observable up to this point.