Heyo, Neth taking over the story forums.
There are many levels on which a character can be identified as compelling. Likewise, there are many people who have different views on what makes a character compelling.
For me, a compelling character is someone that on some level, you can identify with. A compelling character doesn't always have to do the right thing (or isn't even capable of doing so), is seen to have flaws, and mistakes are learned from, not forgotten.
Compelling characters come in wide ranges and spectrums, and are as numerous for heroes as there are villains.
A good example of a compelling character (in my opinion) would be pre cataclysm Varian Wrynn, before he got beat over the head with his demi-god champion status (which as far as I can tell, isn't being excercised).
At this time, Varian Wrynn was shown to be a very passionate leader as he marched the Alliance into Northrend to battle the Lich King. After Bolvar's death at the Wrathgate, Varian Wrynn could no longer tolerate the horde's prescence, and staged an assault on the Undercity, the once great kingdom of Lordaeron, his second home, in hopes of reclaiming it in the name of the alliance and for its rightful owners, the still living citizens of Lordaeron residing within Stormwind and pocketed throughout the region. His retribution was justified and swift when he dove into the Undercity, taking out Putress - A forsaken apothecary responsible for Bolvar's death. From there, he realized that Thrall - leader of the horde - was also in the city walls, and decided it was time to take action. A lot of blood would have been shed, and Thrall and Varian may have well maimed each other to a point of crippling status, but Jaina stopped the fight, and whisked everyone away.
Once the Alliance assaulted Icecrown, it was Varian who appeared and vouched for Varok Saurfang to allow him to collect his son. This was a Varian who dispised the horde, but based on some of the things he had heard regarding the wrathgate, he could give Saurfang this much - a real burial for his son, a true warrior - something Saurfang hopefully won't have forgotten.
This Varian Wrynn was Compelling. As soon as Wolfheart came in, it changed quickly. It turns out that Varian Wrynn's anger stemmed from his Lo'gosh personality brought on by the defusion of his body at the hands of Onyxia. With the assistance of the Worgen, Varian Wrynn not only channeled his anger that increased his power by <insert power up joke here>, and afterwards, he assembled the Worgen, and tore through ashenvale to help the night elves against Garrosh's ambush.
The Varian Wrynn that we have now is not so much compelling. He has little in the way of personality now, and much like Thrall did in Cataclysm, is slowly starting to kick other lore characters in the face while they are already down (Tyrande went from Badass Priestess of Elune to Malfurion arm candy to Varian's inept night elf commander). I liked the Varian Wrynn when his development didn't kill off everyone elses development.
A bad example of a Compelling Character is of course the Mary Sue. There have been a great number of Mary Sue's in Wow's history, and I'm not going to mention the easiest targets, but instead, I'm going to focus on one of the lesser looked at Sues.
Lucan Foxblood is an example of a Mary Sue. He is practically a plot device in order to move the story forward for Stormrage and of course to reach the end of the story, it wouldn't have been possible without him. A human who had no seeming connection to druidism, nature, or otherwise and then unassumingly he is the only person that can do what needs to be done.
Compelling characters can have unique abilities and potentials, but it is more believeable if these special abilities they possess are within their actual family line. While cheesy, if Lucan Foxblood were to learn that his family line had a green dragon somewhere in it, I could have believed it.
Edited by Seebach on 3/29/2013 11:38 AM PDT