Didn't take him too long to get up to speed again, though. I think I agree with the comment that TF1 map code hasn't been fully exploited. TF2 map code allows mapmakers to essentially create their own custom entity code to do exactly what they want to do with a minimum of entities. That sucker is pretty impressive, if I do say so myself. As for adjusting to the new engine, the most difficult adjustment for me was the change in tone.
Quake is very dark and large in nature.
Lots of extreme shadows and lava and dark ominous castles at night. Half Life is much more realistic. The lighting is brighter and cleaner and the textures are more real. I have to bring the scale down to a realistic size or it feels like all the players are small children. This is a good thing.