Designing a successful FPS level is difficult. It's hard to strike the right balance between finding the perfect design, matching up the textures precisely and making your buildings architecturally believable, and getting the level finished and playable. One aspect where the Unreal engines have the upper hand on the Quake-based engines is in the way levels are constructed. Rather than putting objects into an empty space (a la Quake), in the Unreal system you subtract brushes from a space which is initially solid. Oh yes, and the instantaneous build in UnrealEd is handy, rather than having to wait for god knows how long for hlrad to run. One sideeffect of this, IMHO, is the disparity of map quality: Half-Life maps in particular tend to be either extremely good or appallingly bad, with very few in the middle ground.