For a solid general purpose software developer, what specifically is different about game development, either fundamentally or just differences in degree?

I've done toy games like Tic-tac-toe, Tetris, and a brute-force sudoku solver (with UI) and I'm now embarking on a mid-sized project (mid-sized for being a single developer and not having done many games) and one thing I've found with this particular project is that separation of concerns is a lot harder since everything affects state, and every object can interact with every other object in a myriad of ways.

So far I've managed to keep the code reasonably clean for my satisfaction but I find that keeping clean code in non-trivial games is a lot harder than it is for my day job.

The game I'm working on is turn-based and the graphics are going to be fairly simple (web-based, mostly through DOM manipulation) so real time and 3d work aren't really applicable to me, but I'd still be interested in answers regarding those if they're interesting. Mostly interested in general game logic though.

P.S. Feel free to retag this, I'm not really sure what tags are applicable.