it's becoming a very obsolete method of content creation despite it's simplicity.
One of the reasons I prefer source is because it uses brush-based environments. You don't have to learn a modeling program to get stuff ingame, and frankly it takes more skill to do it right. Any half-decent modeler can make something that looks nice, but it takes a real artist to get something looking great using brushes and such a relatively small amount of props. Look at Fallout3/NV's gamebreyo engine; Everything ingame is either a model, or terrain. If you can't make any new models, or don't know how to make terrain efficiently (Which is, at least for me, due to the lack of documentation on the subject) you can't do squat. It gives you all these options to create things, but you can't use them. In source, everything's changeable, and you usually don't need modeling software to do it. (or code, in some cases) Want a new environment? done. Want the metrocops to have pink helmets? Go ahead and reskin it. Source's beauty is in it's simplicity, which far too many people mistake for being outdated. Look at the Dear Esther remake, or Black Mesa, and tell me you can't make anything good looking in source. It just takes the right amount of elbow grease, time and skill.
Jeff: Running on ancient pile-of-crap PC's is not really a good thing
I beg to differ. If your normal PC is in the shop, not with you, etc, you can still play your source games. You can play them at work, although you'd probably get fired. I once saw a guy running portal on one of the crappy school computers. And it does increase Valve's market by a bit, which is why they designed the engine that way when they made HL2.
As computers are increasing in power, they're adding things that they previously overlooked in favor of serving a larger fanbase. I look forward to finally having full dynamic lighting in Portal 2, and I hope they keep adding stuff until Source is once again at the top of the field like it was when it debuted.