Keep in mind, this is not directly taken from any Wowpedia source, but my lore on the Heliustrasz' Felblood Dragons.
Dark Personality: The Curse of the Felblood brings out the worst in the person, a dark mirror of the person they used to be before they drank the blood. All things that could have considered to be a nice trait of personality have vanished. Their dark personality they have harbored inside them have been multiplied by a hundred. Most Felblood are sadistic, enjoys seeing other suffer because of their actions. Most of them are ego-centric, believing them to be the bane of everyone who is not their servant (with the exception of Helius). All Felbloods have their own personality, and there is no set personality all of them has.
"Our greatest evil flows from ourselves"
Enhanced Strength: All Felbloods are mutated by the blood of a powerful demon, most have assumed that the Felbloods we see have consumed Helius' blood. The blood they consumed is the very essence of that powerful demon, and as such they gain a fraction of their power, that would mutate their body. Their muscles have increased expanded rapidly which makes their dragonform most often a hulking dragon. An average Felblood would have the strength to match and most likely to rival an old wyrm's.
Enhanced Stamina: They hardly ever get tired or fatigued, exception would be those Felblood Dragons who use a lot of magic instead of physical stamina. Felblood Dragons can still become fatigued by using too much magic.
Enhanced Hearing: All Felbloods hearing has been heightened by many folds, and if they wanted to, they could tap into this ability, making them heartbeats as a clock put up their ear. This also makes it near impossible for anyone to sneak in a Felblood. Their hearing would let them hear people in invisbility spells (unless hearing is warded against too).
Enhanced Healing Factor: Despite may not be created by the blood of Helius, they share the same aspect that Helius has vowed that would his ultimate goal in life. Felbloods however do not share the true immortality of their master; Helius, but however their wounds seemingly heals a lot greater than a normal dragon's, and as such most of the times, a Felblood would never to be healed but merely get time to the healing factor have its way. A Felblood's healing is seen in the same way as Helius heals himself, a small felfire causes a black steam to emit from their wounded parts and undoes the wounds' existance.
Sire Bond: Felblood Dragons seem to be sired to the source of their creation. Felblood Dragons turned by Heliustrasz has had a deep sense of gratitude and commitment to him causing them to obey his every command. The conditions met to break a Felblood Dragon's sire bond to Helius are currently unknown. But it is known that all Felblood have their own set of personalities, sharing only the common traits of being sinister and sired to Helius.
Felblood Bite: One of the most major traits of a Felblood Dragon is that their bite is venomous, however only really strong dragons (assumed to be those of wyrm agehood) have their bites lead to fatalities. However all Felblood Bites can and will if not the poisen is extracted from the victim cause severe hallucinations and will spread to the whole body. The tiniest fraction of fel venom can expand itself to fill the size of an elder wyrm if given a host to help it procreate. A Felblood Bite CANNOT turn a normal dragon into a fel-dragon (clarifying that Marthamus' case of felblood bite was a special case that was not a normal Feldrake bite, but was heavily modified by exterior forces).
Currently there is no known dragon to be cured from being a Felblood.
Dragonflight-Specific Felblood traits
A Blue Dragon would be more efficient in the usage of fel magic at their disposal, more corruptive and generally the post-spell lasting effects lasts longer than a normal Felbloods.
A Bronze Dragon has the ability to corrupt time with the usage of fel, causing ripples in the fabric of reality that could shatter reality in an area if ordered to, however most Bronze Dragons would not do it on the risk that their own magic would kill them in the process.
A Red Dragon Felblood is more efficient in raising the dead to fight for them. Also it seems their fel magic is quite powerful. It is rumored that the life magic they once possessed is fueling now their fel-magic.