The death of the pc.
New kids are growing up with xbones/ps4's who game on the 40-50" tv's who think of PC gaming as prehistoric. Mom's and grannies who can use there $100 tablet to do everything there desktop can do but much more conveniently.
And as people stated earlier, there is no "killer app". PC gaming evolved the industry for us.
At least from a European perspective, there was a serious increase in PC prices in the last year, compared with not much new on the software front. To give you an example: the entry SP3 used to be 799, the new entry SP4 is 999.
With tablets and smartphones, the software story is actually pretty similar. What was the last great smartphone / tablet app that required a new device?
However, tablet prices haven't been affected quite as much and not everybody has a tablet yet. However, I expect that the tablet and smartphone markets will hit a similar point very soon.