Non-linear designs really get me going. It essentially makes a level seem more expansive, or at least more "realistic." Obviously in real life there are many ways to approach a situation. If you were -really- assaulting a demon-infested moon base, you'd have to decide where to enter, and how to not get lost inside. Levels that are just a series of rooms and hallways don't give you any decisions to make, nor areas that make you want to keep exploring.
Since we're talking about Doom, I must say that, to me, the MOST IMPORTANT part is action. I enjoy wiping out legions of monsters with rockets and plasma, just as well as I enjoy small (but tight) encounters. The real point is, does the action excite me? Almost any fight can be ruined by a few things:
- Being overstocked, or understocked. Being forced to punch guys out can be fun, but it's no good without a rewarding box o' shells or something afterward. Similarly, there's no challenge in holding down your BFG trigger until it stops hitting stuff.
- Knowing exactly what will happen. Dying and starting over usually contributes to the memorization of a level, but even so, enemies can be placed strategically. It's especially bad when woken enemies get clogged up in a corner (map designers must always think of how the AI works!)
- Repetition. I love mindless violence. Pumping possessed humans full of lead will never get old. However, it sucks when every fight is the same, such as "Open door, shoot inside." Or how about "Walk over trap line, turn around and shoot." Even traps can get repetitive and uninteresting.
So those are a few of my to-not-do's. One big thing that I think makes a level great is if the author manages to keep you hurtin', runnin', scared as hell, worried about your ammo, and always checkin' your six, WITHOUT killing you over and over. Of course that's the most difficult balance to strike, but it's worthwhile in my opinion. Some levels you can run through pumping your shotty, others you have to memorize each encounter and execute it perfectly, saving each time. The best levels lie in-between, where you are punished for mistakes but don't feel like you should start over.