So, I was in Ireland for a bit, and one thing I noticed was how well its levels were designed.
There's a strong aesthetic to Ireland which doesn't actually come from the fact that it's green. It comes from the fact that it's got lots of different sized hillocks all over it, and is spotted with lakes and cliffs just for kicks.
As you're driving along - or even just walking along - there is a very strong, panoramic parallax. Much of the time, the stuff in front is large but not totally obscuring the view, so as it parallaxes in and out you gain or lose significant chunks of the scenery behind it.
More interesting to me than that was the way that the roads would hug a hill. Coming around a corner would reveal a whole new vista as the hill falls towards one side.
These are pretty basic ideas, and I'm certainly aware of these kinds of things when I design levels. But not, perhaps, as much as I should be.
Terrain and line-of-sight are very powerful factors in any game, whether you're playing a space marine or a jellyfish or an elf. Recently there's been a tendency to disregard these things - especially in RPGs or MMORPGs... which I think is because this kind of thing takes player skill to handle, and those kinds of games generally don't much like requiring their players to have any skill. But it's critical, it's immersive, it's good juju.
That's why most of the best levels have a mix of open areas, enclosed halls, twisty passages, lookouts, rubble... lots of things to change how you can see and how you can move around the level. Wise designers make the lee sides of walls, doors, and rubble dangerous - that's where smart enemies tend to hide and wait for you. Some games focus on controlling your movement (Sly Cooper) while others focus on controlling your vision (Gears of War).
I guess this is really just a reminder. I actually worked out a lot of formulas and suggestions for various kinds of games and players, but they seem pretty boring now that I'm looking at them.
So... I guess that's it.