Sadness is great. The thing is, S6 isn't sad. It's depressing. There's a massive difference between the two.
Becoming is sad. The Body is sad. Lullaby is sad. Why are these episodes some of my favourites? Because sad is good when there is a balance. Seasons 1-5 are not happy fun times. They are very emotional, very hard on the characters. But good things still happened to the characters. We got a show that reflected real life - because real life is never like S6. It's never that relentlessly, exclusively depressing. There are always good things happening at the same time as bad things.
And just as the happy moments are made so much happier when they come after tough times, the sad moments become so much more emotionally powerful when they come after good times. Look at the last scene of I Was Made To Love You - the stark reality of that cliffhanger is one of the most emotionally powerful moments in the whole show. Do you think it would have been anywhere near as powerful if it had come at the end of, say, Life Serial? The two compliment each other. Remember, light cannot exist without dark, but equally dark cannot exist without light.
Herein lies the problem with S6. Nothing good happens in the entire season. Look through it, there is not a single one. This does not actually make the season sadder, quite the opposite - it makes the season much less emotionally powerful. Drama is all about change, the power of a scene is not exclusive to that scene, it relies on it's context within the episode - ie. its difference to the previous scene. Drama is all about keeping people on their toes. It is the change in tone than is most powerful, not the tone itself. Actually, Lorne puts it best:
"It's like a song. Now, I can hold a note for a long time - actually I can hold a note forever. But eventually that's just noise. It's the change we're listening for. The note coming after, and the one after that. That's what makes it music."
People say that S6 is good, because it's a break from boring old "characters being happy all the time", which is apparently so common on TV. I find this stance odd, to be honest. Yes, characters being happy all the time is boring, but the other extreme is just as bad "“ characters being sad all the time is equally boring. Good television is all about getting the balance there right. Simply, there needs to be both happy parts and sad parts to make each more emotionally resonant, and to stop the show becoming boring and tedious.
S6 is also really quite predictable. The sad moments are best when they come out of the blue "“ look at Joyce's sudden death, or Angel unexpectedly losing his soul. These moments are best because they are unexpected "“ we don't anticipate them, and moments, whether funny, sad, whatever, are so much better when we don't anticipate them. However, after getting 2/3 of the way into S6, we simply expect bad things to happen. Take Hells Bells for example, you could see the failure of Anya and Xander's wedding coming a mile off. It was obvious that they wouldn't be happy, and so that entire episode is robbed of any emotional power whatsoever. It would've been much more surprising, and much more emotionally powerful, if Xander and Anya had actually gotten married. We wouldn't have expected it and it would have made an excellent dramatic change of pace. But as it is, S6 remained predictable, boring, tedious, and above all, lacking in emotional power.
So yeah, that's the difference between sad and depressing. That's why I consider the first five seasons to be wonderful, harmonious melodies, while S6 is just noise.