As I have mentioned, I've been mapping for Unreal and UT SP. UED1 is fairly powerful with irritations, still can do things not possible in Quake. UED 2 is much nicer but only works for UT; I'll probably switch to pure UTSP if I continue. I'm using an old engine + modern PC to extend capabilities rather than something newer which probably couldn't do what I want.
Thing about singleplayer mapping: NOTHING about the game is important compared to the quality of the monsters/creatures/enemy. This rules out, or works against, practically every newer game in existence. Q3 nothing, UT2k3 may be able to use U2 stuff but is that worth much, Q2 somewhat lame and unmemorable, Cube Ok but basic, HL, RtCW and AvP limited types (with AvP 1 all but unmappable), Sam very specialized combo of suiciders and stand-back-and-fire-ers. Some decent SP can certainly be made with these games but the diverse behaviour of Quake enemy remains attractive. Even the complex behaviour of Unreal creatures lacks the more blatant variations in Quake.
More to the point other than Quake's age is that it's just been worked over so thoroughly. Other games could do some great new things if they got that amount of attention and work. OTOH, the over-saturation of Quake leads to a superior community, loyalists to less popular games sometimes seeming like Mac fanboys.
Incidently, a new project came out due to the fact that UT is both a big DM game and supports SP: Excessive Unreal (
http://exu.netfirms.com/ ). The only SP Excessive game. NOT traditional SP (uses recharging) and pretty ridiculous but still a lot of fun.
I'm still looking for beta tests for my new serious-but-crazy Unreal level, report to
http://www.intergate.com/~spentron/unreal/screens.html to help out.