I have known since well before TF2 that decompiling half-Life maps made a mess, and I had heard that decompiling is a lot less messy now, but I had just figured that the algorithms had somehow gotten better. Thanks to this thread, I finally know what has changed: the bsp format now includes the whole brush, and not just the visible faces, which obviously would make decompiling much easier.
Thanks! I like learning new things.
Also, if you somehow manage to import a goldsrc map into TF2, you'll find that the scale is very different. Everything would be way too small for the TF2 characters.