I can tell you why the valve games are reviewed well.
L4D1/2- These games have a lot of content. If you play the game alone, on easy, then maybe you could beat it really fast and feel like there wasn't much. But if you play on advance or expert with 4 friends you can spend hours on just one chapter. And with the director it feels like a different experience most of the time. Then there is the vs multiplayer which is incredibly balanced for such a difference in gameplay. If your the type of player who enjoys competing completion times there is survival mode. Plenty of content.
The biggest reason the left for dead series scores so high is because its a game with co-op that doesn't feel like it would work without it. The entire game is designed with 4 players in mind. And uses that greatly in its design. Using a recent example, Resident Evil 6 would play no different, it's design wouldn't even change, with the exclusion of co-op. In l4d if just one survivor dies you feel the impact immediately, and the game becomes harder.
The Portal series- The first Portal game was perfect in its length. So many developers fail to understand that adding more hours to a game does not always make it better, and you seem to think this as well. Most of the time it makes the game worse. Anyway, the reason portal scores so well is its very close to being the perfect game. Most people went into Portal expecting a puzzle game and nothing else. But what they got was one of the best video game stories told, using play as the method for telling it.
The design is completely hidden from the player. Where games like call of duty tell you exactly what the designers want you to do, Valve used Glados. The tutorial is completely hidden in the form of Glados' narration( which could be said for the entire game. Even using her betrayal as a way of telling the player, she isn't helping you anymore, don't obey her.) At no point do the designers just give up and tell you exactly what your supposed to do, they are completely confident in their design.
Portal 2 had a lot more pressure than the first ever did because of the incredible word of mouth the first had. I think the reason it seems easier is the shift away from the test chambers and moving into other settings. Which forced them to use the white panels as guides (having every surface portal'able in the Cave Johnson sections would make it too hard for most players). Which helped players get a general idea of where the portals could be placed but that also meant for the more skilled player, they would figure out the puzzles a little faster. Now I would love know how many players beat the game well below the average time completion ( first playthrough) because I would imagine its not by much. Which would prove that it wasn't that much easier than the first.
The reason the 2nd reviewed well is because its a great game, which you prove by only listing difficulty as a negative. Every reviewer might not be as good or as fast as you were, or maybe the reviewers value difficulty differently.
This is becoming a bit too long for me to do the Half-Life series. But if you enjoy talking about them, PM me and I would love to hear your thoughts.
tl;dr Valve games review well because they are masters at game design.
