Have you ever noticed that there are very few areas where true hight exists, where you can run over an area where you have already run over? these are in Newland trade shops, there is an upstairs, and in staying alive there is a bridge where you can watch two girls stare at each other. another is if you fall off a bridge into the water, and then swim under where you fell off.
The problem is that most of us have played various 1st and 3rd person shooters, and are used to aiming up and down, gettinglost in multiple level dungeons, sniping from safe locations, enjoying gravity's affects on grenades, climbing walls and stares, firing at truely flying creatures, and crawling past guards through sewers.
I wonder how much it would cost to license the quake1 engine now. I would gladly pay more to improve combat.