I say they are both about the same, HL1 you can impress people with simple brush work and the low level lighting. Custom "anything" is simpler (textures), but they don't look as good with out that nifty bump mapping.
While in HL2 you can imerse your self in the world and create a realistic picture using displacements (Their aplication is famonimal)/water/materials/physics. The only problem I see is that people are using the same props over and over again, it gets boring after a while. What needs to be done is have a way for model/material creation to be simplified.
Also the HL2 community is filled with new mappers, and all we see is boxy rooms with weapons/props slewn everywhere. HL2 mapping hasnt quite taken hold yet, but if given some time is might rise to the level that HL1 is at.
If anything im worried about how DM is practically dead, there are many reasons for this, some on valve's part, but a lot on the HL community as well.
As for the site, I think it isn't what it should be, interlopers has weekly tuts (alomost) and they are more centered around the mapping in general, while this site is more comprised of critiquing the maps that are submitted, and that scares people away, because of the harsh treatment everything is given.
Id also like to point out that when HL2 was released that the member number rised from 800 to 2200 in about 7 months. I ask myself if these 1200 members are contributing to the site (I know there are many "regulars" out there like Satchmo, Dr. Glass who know what they are doing), but there is just a surplus of people here not doing anything to help the site (me included). (If anything I miss the old days of when this community was more compact)
Id agree to another full time admin to work on the site, only problem is I know no one who is as good at PHP as lep is. I think that sufices as to whats been commented so far. More observations please.