To start at the beginning, I used to feel ambivalent about the Forsaken. I didn't like them enough to play one of my own until I rolled my death knight, but I enjoyed their company and liked to spend time in their zones. They always struck me as the Azerothian equivalent of that misanthropic guy in your circle of friends whom you suspect you'd really dislike if you got to know him more closely, but as long as you keep him at a safe distance he makes for great company, because he's also clever, sarcastic and funny.
Looking at it a bit more closely and seriously, the Forsaken have always been giving quests that were considerably more evil than those that you got anywhere else. Yes, all factions ask you to kill people, but the Undercity was the only place where they made you think that you were actually helping the one you were going to get killed and considered this perfectly normal behaviour.
And yet, despite of this, I could never get myself to truly dislike them, probably because I was also feeling a bit sorry for them. They are not like other races, they aren't even really a "race" at all. They are sentient abominations, forced to exist in a sort of limbo between life and death where they are unable to truly enjoy anything and have no real purpose in
Wrath of the Lich King was a big expansion for the Forsaken, because it went back to their roots and gave them purpose, reminding everyone of how it was the Lich King who was responsible for their current plight and that it made perfect sense for them to want revenge. But then the Wrathgate happened... and it was painful. I'll never forget the shivers that ran down my spine as I watched the cinematic for the first time and saw Putress appear, threatening Arthas with the wrath of the Forsaken (yay, here come our crazy but ass-kicking allies) - until he added "and death to the living" to the end of his speech (oh shit).
After that I felt that things kind of went downhill for the Forsaken. Before that I had always considered the apothecaries a sort of extremist group that wasn't necessarily representative of the undead as a whole, but the Wrathgate made it very clear that they were indeed the ones in charge. Sylvanas denied all responsibility afterwards, but I challenge you to find anyone who actually believed her story.
Now the Forsaken weren't just poor disgruntled monsters anymore, they were traitors. While it had been easy to feel at least some sympathy for them in the past whenever they seemed uncaring or unnecessarily cruel, this was personal, outright treachery and simply inexcusable. I was sad about this because it felt to me like this development really eliminated a lot of shades of grey from their character as a race and just left them as this purely evil people that couldn't be trusted even by their own allies.
As such I wasn't actually looking forward to seeing how their story would develop in Cataclysm, especially after I had heard reports about Sylvanas going mad with power and effectively becoming the new Lich King.
Let's just say, the new undead starting area was surprising.
Tirisfal Glades not so much, as it's retained a lot of the old starter quests; the experience has just been smoothed out considerably. Yes, there are some new quests and they are fun, but nothing that struck me as really out of the ordinary. The only thing I found notable was how different the reception of my new undead character felt compared to the old starter zone. Pre-Cataclysm, it basically said to a newly risen Forsaken: "Oh, you woke up too? Sucks to be us, let's try to make the best of it." Nowadays it says: "Welcome! We brought you back to life so you can serve the wonderful Lady Sylvanas!" The atmosphere is almost... friendly, like you're being inducted into a special club.
When you enter Silverpine, that's where things get really interesting. You immediately get to witness Sylvanas explaining her newest scheme of having Val'kyr intentionally raise new Forsaken to Garrosh, and her choice of words is fascinating. "I have solved the plight of the Forsaken," she says, and "as a race, we Forsaken are unable to procreate". I've seen people brush this off as her simply wanting to "produce" more soldiers for the Horde war effort, but to me those are not the words of a warlord talking about her cannon fodder. You want your people to procreate, Sylvanas? That sounds awfully... maternal.
A bit later you get to ride side by side with her as she explains the history of the Forsaken to you. Usually when your character gets to interact with an important NPC, this strikes me as a way of trying to make you feel more important: you're such a great hero that even the Warchief himself (or whoever) comes to have a chat with you. In this case I got the opposite impression though, namely that this conversation showed that Sylvanas is a leader who genuinely cares about her people, even a lowly schmuck like my level twelve hunter.
In a later quest, you get sent on a rescue mission to save the survivors of an ambush. Rescue mission? What? Are these the same Forsaken that asked me to mercilessly kill any of their number that were unlucky enough to get captured by the enemy in Dragonblight?
During another quest, you accompany a group of soldiers into a cave where they end up getting ambushed. Before I had time to properly process what was happening, the leader of the group shoved my character out of the blast radius, with his last words being that I should tell the Banshee Queen. The way I sat in front of my monitor in stunned confusion must have been a pretty good reflection of the way my character must have felt about this strange act of heroism. "But... I'm just the rookie, and yet he bothered to save me. I don't understand."
Don't get me wrong, I'm not saying that the Forsaken have suddenly turned into a bunch of fluffy bunnies. They are still ruthless and cruel, but something has changed for sure. They don't just loathe everything and everyone anymore, including themselves. They stick together. They care about the survival of their people. Up until Wrath, it was all about them killing and bombing their way towards the Lich King, no matter the cost, but with Arthas dead, what was left for them to do? Lie down and die?
It seems that Sylvanas has decided to make the Forsaken a "proper" people. They are still undead and messed up, but they also want to have their own land, procreate and get some joy out of their existence, even if it's just from worshipping their Dark Lady. This is a huge change in my opinion, and one that makes them a thousand times easier to relate to. Sure, you can still hate their methods and that's fair enough, but at the end of the day their goals are now not so different from those of the other races. They just want their people to flourish, in their own undead way.
As if to drive the point home, the zone ends with you having to work with the new bosses from Shadowfang Keep for a while, and those guys are bastards. They combine all the worst traits of the old Forsaken philosophy, loathing not just their enemies, but also their allies and even themselves. During the aforementioned rescue mission, Lord Godfrey sometimes randomly pulls out a rifle and shoots the soldiers you just saved because he thinks that they are worthless. This is perfectly in line with old Forsaken quests (refer to the one in Dragonblight I linked above!) but as a player of a "new" Forsaken you can't help but hate him. You're being taught to care for your fellow undead, even if you don't care for anybody else, and you just don't treat them like that!
Later in Hillsbrad, there is a similar situation where you encounter a crazed apothecary for a while who has clearly gone off the deep end and is raising mindless zombies everywhere. I immediately felt uneasy when I saw his whole operation and started to wonder whether I had overestimated the "goodness" of the new Forsaken... until I found Master Apothecary Lydon locked away in a cage and together he and I went back to kick some butt and clean up the mess, because again, this was actually not acceptable by the Forsaken's new standards.
I certainly didn't expect the Forsaken to come out of the Cataclysm more likeable than ever, but there you go. This is my interpretation of their quests at least. I get the impression that a lot of people seem to think that Sylvanas is still scheming quietly about how to destroy all life on Azeroth and how to become the Ultimate Queen of Uber Evil, but I have to admit that I have trouble seeing that side of her, going by the way she behaves in game. Yeah, she hates Garrosh, but who doesn't? She is genuinely saddened by the loss of her Val'kyr companions, and when she deals with Crowley at the end of the Silverpine story, she honours their agreement to let his daughter go unharmed. I think that she just wants to see her people prosper, and while she definitely still has a bone or two to pick with certain people, I doubt that she's hell-bent on world (or even Horde) domination.