For example:
A nice looking tree mesh can be made by a decent modeler with ~1,000 triangles. The same tree mesh could be made by a novice with ~5,000 triangles. So, if it's a well made mesh, you could have 10 trees for a similar performance hit and appearance to 1 tree. For example, Skyrim's trees often use 3,000 triangles, so sort of in the middle. Dark Souls IIs' trees use about ~700 triangles and look just as good most of the time, which is one of the reason Dark Souls 2 is much better optimized than Skyrim (looks better and will run at 60 fps on a GTX 650 on ultra, unlike Skyrim, which will take a GTX 760 to run on 60 fps at ultra despite looking a bit worse).
If a game uses 512x512 filtered shadows, it'll look just as good as a game that uses 4096x4096 shadows without filtering. Sorry to pick on Skyrim again, but it's a great example of a well known poorly optimized game. Skyrim uses 4096x4096 shadows, which is why they murder framerates, but the shadows aren't filtered so they still look blocky and often bad. Most games' shadows look better, and the standard right now is 1024x1024 per shadow.
Aside from that, we have to consider how a game uses resources. Some games are meant for many CPU cores, some are meant for very few. Dark Souls II is meant for 4 CPU cores, which is likely to be the standard through this generation. It runs well on AMD CPUs, and amazingly on Intel CPUs as a result. However, some games, like Skyrim, are mostly meant for 2 CPU cores. Extra cores go unused, making it run very badly on AMD CPUs and only somewhat well on Intel CPUs.
There's also memory use to consider. Watchdogs in particular is a port from the PS4. The PS4 uses a strange architecture where they have a lot of VRAM, but very little system RAM. Because of this, Watchdogs was coded to use almost entirely VRAM, not system RAM. They didn't change this properly when porting it to the PC, so it's badly optimized because of the ridiculously large amount of VRAM it needs, when it really shouldn't need that much.
It all goes on like that for a hundred different things.
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