More tl;dr stuff:
The red down-arrow that used to show unhappy dwarfs now shows dwarfs that are "under a great deal of stress." (Not sure if there are other negative states that will also have it). The line that says they are under a great deal of stress is the very first thing in their description, right after their active thought, and is in yellow.
The main difference between this and unhappiness seems to be that it's no longer just a matter of "give them more positive thoughts than negative and they're okay again." Stress seems to be more of a long-term thing, and they can still have only a few negative emotions and more positive ones in their current/within the last season thoughts, and still be stressed. So I have a suspicion prevention is going to come into play a lot more.
Also I'll note that most of my dwarfs seem fairly resistant to it - even after two sieges, where I've lost probably around a fourth or more of my fortress at the time, and a few other mishaps, it's nearly only dwarfs that are describe as "does not handle stress well" or "cracks easily under pressure" that are stressed out in my fort at the moment. There are two exceptions that I've seen:
The first is my Captain of the Guard, and that could be partly due to the fact that Lol!noble, and partly due to the fact that she has extra stress (frustration specifically) because of my lack of chains and cages. She also has the "easily buffetted by others' emotions" description, which may or may not be playing a part.
The other exception is my one and only dwarf that went mad, but this was because they were wounded, and needed treatment, but weren't resting, and I couldn't seem to get them to rest whatever I did. So they kept getting more and more stressed out due to lack of sleep and such, until they finally snapped, and wandered around until dying. At that point, they had the description "has been overthrown by the stresses of day-to-day living" in magenta font at the start of their description.
I'll also note that I didn't notice any negative emotions from others when they say a crazy dwarf wandering around. When the dwarf finally died, their relatives (children, siblings, etc.) got the dual emotions of being shocked at an unexpected death, and grieved at the death of someone. Also afraid after experiencing trauma, and I don't remember if that last one was triggered just by being around a crazy dwarf suddenly dropping dead near them, or the fact that it was a relation. Sorry. :/
I've also noticed one other extra message - "She doesn't really care about anything anymore." This is found at the end of the description, right before the "A short, sturdy creature fond of drink and industry." After that point, she just felt nothing at seeing other sentients die, instead of the usual "horrified." Sadly I didn't write down who this was, like an idiot, so I'm not sure if this is a temporary or permanent state.
As one other side note, I still haven't been able to figure out just what causes some dwarfs to take pleasure near nice furniture (or roads or trade depots or whatever), while otehrs have are interested. I haven't yet seen one dwarf react with different emotions to admiring things, so it doesn't seem to be a matter of the quality of the furniture or building. It seems to be a matter of personality, but I can't quite pin it down. Dwarfs that admire art, or craftsmanship (which seem to be different things) seem to come down on both sides of the pleasure vs. interest spectrum, likewise I haven't found any one trait common to the dwarfs on either side of it. I have a feeling this might come down to taking screenshots of dwarfs on both sides and settling in for some serious compare n' contrast. -_- About the most I can say is that SOME of the "interested" dwarfs admired art and artists, and SOME had personality traits like "optimism" or "does not easily hate or develop negative feelings," or maybe a job related to crafting things (like masonry), or some also had "dreams of creating a great work of art"... but again, this doesn't seem to be found in all the "interested" dwarfs.
As for more trigger/emotion combos I've found (and colors if I hadn't seen/given them before):
Adoration - after becoming a sibling
Agitated (brown) - when utterly sleep-deprived. (This was the aforementioned hurt-but-not-resting dwarf.)
Annoyed - being caught in the rain
Content - eating a pretty decent meal, after a soapy bath. (Note that the "bath" only seems to be if they're actually cleaning themselves. I set up "dwarven bathtubs" that some of them had to walk through, and they had no emotions after that.)
Disgusted - being nauseated by the sun
Embarrassed - after sleeping without a proper room
Exasperated (brown) - when caught in the rain. (This is different from some dwarfs' reactions to rain, and might be because this dwarf was described as "quick to anger.")
Exhilarated (blue) - after being attacked (this was my soon-to-be-dead previous Captain of the Guard, who was "brave in the face of imminent danger." Not sure if it was a matter of office or personality.)
Free (green) - after being released from confinement
Frightened (yellow) - after being haunted by her own dead child. (Eesh. Also note that this is different from "afraid," which is in red.)
Frustrated - after being unable to find somebody in charge to yell at/cry on
Gloomy (brown) - at being out in the sunshine again
Gratitude - after receiving water, being rescued
Grouchy (brown) - being caught in the rain
Guilty - after being confined (In this particular case they weren't imprisoned for a crime, they were accidentally caught in a cage trap. I built the cage before I noticed it, so not sure if they'd get the guilt just being in the cage, or if it's only a built cage, such as in prison.)
Horrified - after seeing "a dwarf" die, not sure why this particular one wasn't named. Also after seeing things as simple as troglodytes die, so yeah, pretty much any sentient creature it seems.
Jovial (green) - after having a mandate deadline met
Love (aqua) - after gaining a sibling, talking with mother
Nothing - while in conflict, which is different from some dwarfs.
Pleasure - after a sparring session
Satisfied - after producing a masterwork or creating an artifact
Self-pity (brown) - at being out in the sunshine again (note that some dwarfs were "gloomy" at this trigger)
Shaken (yellow) - after suffering a major injury
Tenderness (aqua) - talking with a child
Terrified (red) - while in conflict
Vengeful (red) - when joining an existing conflict