I've heard that Shadow of Mordor is not as good on the PS3 as on the newer PS4. Graphics aside, are there any differences in gameplay between the two consoles?
Really the only differences is the somewhat more simplified Nemesis system. Basically, the system is more limited on the PS3/Xbox 360 versions, because it apparently relies on some quite heavy content, calculations and AI.
I have not been able to find a simple comparison of what's missing or simplified. However, it should come down to the system feeling a little more stale, with less variety and less complicated events unfolding.
As quoted from an interview with the design director Michael de Plater:
What it won't have is the same level of depth and variety and simulation within the 'Nemesis system'.
"The story will be the same and the core gameplay will be the same, but [the 'Nemesis system' is] just so huge in terms of content, calculations and AI we'll just have to try and get as much of it in as we can."
I haven't played the PS3 version myself, but I can hardly imagine it would be so bad to skip buying the game. However, it does seem that the (few) game critic sites that have reviewed the PS3/X360 versions specifically, do consider it quite badly. Take a look at this review, for example.