Not only is she a hasty 6/5 flyer for 6 mana, but Kolaghan also gives your other creatures haste, as well. The most interesting thing about her is the last ability: if an opponent casts a creature or planeswalker spell that has the same name as one already in that player's graveyard, that player loses 10 life. That's actually quite a powerful ability that's perfect for the top end of a control deck.
Alas, a spot for her never appeared in the Standard metagame. Famous Magic deck brewer Conley Woods built a Mardu Ever After deck with one copy of Dragonlord Kolaghan in the main deck. That deck used the sorcery Ever After to return two creatures to the battlefield from the graveyard, in this case the Dragonlord and perhaps Linvala, the Preserver or Archangel Avacyn. It was a solid plan, but the deck never really caught on. Basically, Dragonlord Kolaghan just wasn't worth casting very often in Standard, despite popping up here and there.
Oddly enough, the format that has adopted her the most is Commander, the one in which her final ability is irrelevant. Basically, it's tribal Dragon decks that just want her to grant haste to everyone, and being a big hasty flier herself doesn't hurt.
Despite apparently not finding many homes, there was still demand for her as the Standard rotation of Dragons of Tarkir approached. Even as her retail price has dropped to nearly $2, many buylists still paid close to $2. That's due to plenty of casual appeal to this Dragonlord. She is a pretty good reanimation target, albeit not in other competitive formats.
While she may not be the greatest choice to lead an EDH deck, Dragonlord Kolaghan decks do exist. She's probably going to remain a casual hit for years to come, so don't throw her into the bulk mythic rare bin.