As the title suggests - If you're currently an owner of a high end gaming PC and are used to most games running at 1080p or higher over 60fps, you're really not going to be blown away by the next PlayStation and Microsoft consoles (I'm now refusing to refer to them as 'next-gen', for reasons i will explain later).
Essentially, games are going to run at a higher res, smoother (more often 60fps) and finally, in actual hd resolutions, rather than the sub-hd seen on the ps3 and 360, hopefully most of which will run in full 1080p without trickery, But this is something high end PC owners have been doing for years, on hardware much higher spec than the 360 and PS3
Take the following 720P image for example : http://i.imgur.com/ILlQ6AQ.jpg
It's split three ways across the entire image multiple times, showing the graphical differences between the PC, PS3 and 360 versions of Split Second, quite an old game now but still dramatically better on the PC.
So why don't you want to call the PS4 and 720 "next gen"?
Well, throughout history consoles have provided a decent amount of power compared to PC's of the time at release, the unified configurations and game specific hardware with no driver or os overhead allows hardware which wouldn't be very good in a PC to run much more efficiently - the problem we have at the moment is that the GPU and CPU designs that form the basis of the next consoles is by high end PC standards, already extremely slow when compared side by side against high spec gear - so even if we assume the hardware difference makes the next consoles on par with high end pc's which is in itself a little far fetched - the overall difference between the 360 and PS3 and current PC titles is a fair jump but mainly just resolution, framerate and differences such as anti aliasing and texture filtering, so while things will look better - if the next consoles can only manage the current graphical quality of high end pc's or near to it, people who own and use high end PC's aren't really in for any major shocks as far as graphics go, and will just come away thinking 'okay so the game runs at 1080p 60fps on the console, but 1600x1200 at 90fps on mine..'
So the real kicker is this, if people argue wether or not the WiiU should be labeled "next gen" or not, because it's "only just catching up with the PS3 and 360", when the other "next gen consoles" are being released soon with more power - then why should we also call the PS4 and 720 next-gen if they're "only just catching up with pc", knowing full well the PC hardware market increases in power almost monthly - with major hardware family releases almost every year, and the new GTX7xxx series and AMD GPU on the horizon too, would it be fair then to say that for truly 'next gen' games, we should look towards the PC?
I'm fully expecting a lot of butthurt Sony and /Microsoft fans venting at the thought of their next platform that hasn't even been revealed, be reffered to as 'not next-gen' or 'outdated', but that's basically the long and short of it.
If you don't want to call the WiiU next gen, don't have the audacity to call the PS4 or 720 next gen either.