The original Portal game is where I started to get into 3D mapping and the Source engine. After 6 or so years of mapping for Portal, HL2, L4D, etc; I always go back to Portal/Portal like projects due to these reasons:
1) The map geometry is the most simplistic out of all Source, and perhaps most current games out there. You don't need to worry about advance detailing if you don't want to. This makes Portal/Portal 2 a much more welcoming game to start learning the source engine then HL2 or TF2.
Sure, in Portal 2 you can do the vegetation, destruction, and all that fancy stuff. But to make a basic Portal map, all you need is a few walls, a ceiling and a floor at the very least.
2) It's (mostly) a Single-Player experience that people enjoy and may go back to. I never saw a reason to map for HL2, or a desire to download custom HL2 maps. When people did HL2 themed stuff, it was usually a full mod. However, Portal is one of those games that beg for user generated maps. Even if you look at the X06 demo, the Portal UI has a slot for "CUSTOM MAPS" which turned into the "BONUS MAPS" section. Valve knew this would be a game that people would want to make maps for, and share with others.
You also don't need to worry about the stuff you would need to worry about in TF2/Counter-Strike/whatever, and worry about team spawns, weapon pickups, round times, etc. If you are doing co-op maps for Portal 2, the rules stay the same as if you were doing singleplayer. You would just need to think with another set of portals.
3) Again, the simplicity. Seriously, the simplicity of Portal is genius.
4) Ok, this does not apply to Portal 2 as much, but something that was kick-ass in Portal was that with just one map you can release a advance and challenges with the original map. It was one map, but you could be given two, and three sets of challenges to face. I was never good at setting these challenges, but the idea of having them there is what I really miss.
My least Favorite aspect is that you're map will never be for everyone. Sure, you can disagree with me here, but there are people who like complex puzzles that look like chaos at first glance, and other people like maps that they can play though fluently, and still have a ton of fun. Then you have the remaining people who like those fling puzzles you use to see in Portal 1 maps such as the side fling and crouch fling. (i.e Play Logic Portals. Especially 04.) Pretty much, just make type of portal maps you would want to play, cause in the end someone is gonna have a problem with it anyway.