While I agree we don’t get the physical/living “rush” of emotions/hormones, I’d say it’s a stretch within the lore that we feel nothing except revenge.
Sylvanas herself still felt great love for her sister Vereesa, in the War Crimes novel. Sure, it was twisted and a little off (she wanted to make Vereesa dead), but she felt love. Beyond that there’s Thassarian, a death knight who still loves and protects his sister, and feels a strong connection with fellow Death Knight Koltira Deathweaver. If anything it’s not that the feelings are gone - it’s that they’re muted somewhat. For some more than others.
Furthermore, with free-will comes the capacity to reason. The whole “fake it till you make it” thing is relevant here - Sarestha for example, I RP as having a strong moral compass for the most part. She actively tries to fight against the cruel aspect of her nature as a death knight, by deciding what is right and wrong. Even in the absence of strong feelings, she has a recollection of what morality is, and chooses to abide by it. The undead not having feelings thing is proveably false in lore, and it’s been that way since at least BC, when Sylvanas mourned for Alleria.
Many of them, including those that the ‘adventurer’ plays as, seem to be actual Lordaeronian corpses, exhumed from graveyards. Lillian Voss, Valdred Moray and Marshal Redpath (the three Undead we encounter in the starting zone) were all Lordaeronian, although two of them were brought to Deathknell from other parts of Lordaeron - Silverpine and Hillsbrad respectively.
With this in mind, I would argue that evidence suggests that most Forsaken are still Lordaeronian, although there’s no doubt that Gilnean, Stormwinder, and even Stromic Forsaken do exist. Hell, we even have Kaldorei now. But the foundational masses, the majority of the Forsaken, are Lordaeronian.
The Light, when used for healing, has never been harmful towards Undead. Sure, they have a weakness to it. It burns them, it hurts like hell, but it doesn’t actually harm them. Here’s some quotes from the wowpedia article about the Light: https://wow.gamepedia.com/Light#On_undead
When undead channel the Light, they do not disintegrate or explode from channeling the Light, though they may wish they would. Instead, it feels to them as if their entire bodies are being consumed in righteous fire. Forsaken healed by the Light (whether the healer is Forsaken or not) are effectively cauterized by the effect: the wound is healed, but the healing effect is cripplingly painful. Thus, Forsaken priests are beings of unwavering willpower. Forsaken and death knight tanks suffer nobly when they have priest or paladin healers in the group, and Sir Zeliek really hates himself.
There are reports that some Forsaken have slowly experienced a sharpening of their dulled senses of touch, smell, etc., as well as an increase in the flashes of positive emotions that have otherwise become so rare since their fall into undeath. Unfortunately, this may be the cause of the Forsaken priesthood’s increased attempts at self-destruction; regaining these senses would force the priests to smell their own rotting flesh, taste the decay in their mouths and throats, and even feel the maggots burrowing within their bodies.
I actually find this to be pretty cool. The Light does have a healing effect on the Undead, to the point where some of its users actually regain aspects of their senses that they had in life. It’s unpleasant, but it heals.
With that in mind I don’t actually despise the notion of Lightforged Undead. The exact nature of it and how it affects Calia intrigues me. She’s an anomaly to say the least.