I don't know why EVERYONE loves dm1. Personally dm2 and dm5 are my favourite out of those 8 maps. But that's just me.
dm1 has some cool dynamics to it, it has a LOOOOT of cool jumps, its a classic
it also works great for ffa, 1v1, 2v2, 3v3, and 4v4
its kinda like monsoon on q4, or dm6 on q3/ql.... its just the goto map for quake2