For a long time, Glorfindel was the go to glue hero for a deck. Low threat, great stats, the whole shebang. While he still serves that purpose extremely well, Denethor has replaced him for me.
While obviously very different roles, Denethor and Glorfindel both are used for their early game upside: low threat or free resource. (I won't compare these two any further, as they have more differences than shared traits, but I think that it's worth mentioning.)
Denethor's ability is quite possibly the best resource acceleration in the game not called Steward of Gondor. Of course, it's a one time thing, but with two extra resources turn one, the amount of things you can do is crazy. Drop Steward of Gondor on a hero, then play Gandalf, on your first turn, for instance.
His second ability is less amazing. Of course, it still has it's uses, but they are far less powerful than two free resources.
His stats are fine. Three defense and three hit points make him an okay defender, but nothing more. The threat cost is low enough to be thrown into just about any deck.
Last but not least, the art is stellar. This is up there with some Magali art in terms of how good it is IMO.
All in all, Denethor is an excellent hero who should get more love then he does.
Final Verdict: 8.5/10