That is because with this generation of console is focusing on frame rate @ 1080p / 900p. You'll notice in a lot of press releases they are talking about maintaining 60FPS, whereas with the last generation the were talking mostly about visuals. This is why a lot of upcoming PS4 and Xbone games look almost visually the same as their predecessor because they can't do significantly better visuals and maintain 60FPS @ 1080p or 900p at the same time. Yet.
To a degree, yes, with very high resource multi plat games that have large worlds like AC IV, you're going to see some noticeable tradeoffs compared to the PC version, but when you look at exclusives like Killzone Shadow Fall, it becomes obvious that this gen also looks much better than last.
This gets back to what I was saying, that any system is only as good as the quality of authoring. Exclusives always get extra graphical polish and optimization. It was the same with last gen. That said, since exclusives are now (so far anyway) a much smaller percentage of available content, the power of the system becomes less of a factor.
One thing that hasn't really been touched on as far as how Steam Machine compares to these next gen consoles is that in using standard PC hardware, it will be MUCH easier to write for. I don't care about fixed hardware on consoles, when they have numerous processor cores to handle both CPU and GPU duties, it becomes more complex to write for them.
This is why multi plat titles on consoles often look mediocre. Sony and MS like to brag about what these systems can do, but in reality you usually only get the benefit of their full potential on exclusive titles. In going with an APU they economize, but it definitely shows up in a lot of titles.
This is why I don't pay much attention when people say consoles already benefit from fixed hardware as far as development efficiency when Mantle is discussed, because we've all seen numerous times that there's a LOT more to making a system easy to write for than fixed hardware. The hardware design itself makes a huge difference.
Too many devs and pubs are toeing the line for Sony and MS trying to convince players that PC development gets pushed back due to the added complexity and testing required, but it has more to do with prioritizing their cash cow console systems that garner 80% of title sales.
