Here's my two bits. But first:
Holy shit, teh jobabob. Welcome back.
Anywho.
I've used the SourceSDK extensively when it came out and I've recently returned to Hammer 3.5 to map for a HL project. In my opinion, comparing Source to Goldsource is just petty. More than likely, you, as well as ZombieLoffe have gotten used to the way Hammer 3 runs, and mapping in general for Goldsource (which is actually extremly easy if you've been on Source for a while). As such, adjusting to a new engine that brings with it so much more complexities that Goldsource never had (props, physics, etc etc) is obviously going to pose more of a challenge and thus be more frustrating if you don't give yourself time to ease yourself in.
As for the SourceSDK system and Steam in general, its another petty arguement. Steam is a huge, HUGE platform, delievering content to God knows how many PC's. Can you realistically expect that it will work flawlessly on every single PC that its installed to? I think the fact that Valve constantly update it shows that at least they care (to some level) about their customers and are willing to invest time in a gradually improving piece of software. Personally, I've had absolutely no problems with Steam whatsoever: updates are easy to get, Hammer automatically configures itself, there's no extra downloading of bits and pieces to do, its all there and ready to use.
As for Hammer itself, I find myself missing the MANY improvements that Hammer 4 has over 3. Even simple things like transparent textures is something that I've grown to rely on.
Perhaps I got relatively lucky: I picked up Source mapping relatively quickly. But really, if you're gonna complain about silly things (not nessecarily you, jobabob, but some members of the mapping community) like the fact that you have to use prop models, then you're an idiot. Please explain to me how you would insert some decent detail into your maps, such as trees, little props, etc by using brushwork? Come on people, Source is a modern engine, you can't curse it because it wants to make an attempt at looking decent!