I'm not saying that based on Eccleston's performance, obviously. I'm sure he'll do fine in the role, even if he downplays his own role and performance in interviews. I'm saying that based on Malekith's actual role in the Marvel Comics universe and the cinematic one. Malekith is a very minor Thor villain (talking importance here, not power levels, so no e-peening "my villain is more powerful than *your* villain" dick-measuring here). His primary role in the comics is to set up the fan-favorite Twilight Sword saga, which sets up a more popular, more powerful, more frightening and more effective Thor villain: Surtur. That's most likely going to be the case in the MCU as well.
Plus, Thor is not going to focus all his attention on one villain. Malekith is one of many problems Thor faces in this movie. There's also Kurse (Kurse is *not* on the same page as Malekith, and in fact battles him in the comics); Loki; The Marauders; a possible love triangle with Jane and Sif; more family issues with Daddy, Mommy, and Lil Brother. And that's just the ones we know about....there's probably other conflicts that have yet to be revealed. The "main villain" of the movie is *not* going to be Malekith, but the "dark ancient force" he's summoning, and which lends the movie its title. Comic-book canon says that's Surtur; could be Thanos; could be Chthon; could be something else. In any event, it's about a much greater threat. Malekith is just a harbinger.