Wasn't he the lead actor in Django?

He really isn't the lead, the movie is'nt "King Unchained", it's "Django Unchained".... he is supporting Django, who comes out as the hero in the end. I loved his performance, though, and hey... he's getting the praise he deserves, so it's all cool. He still has many years ahead of him to wow us with lead performances.

Title doesn't mean automatically lead. E.g. in Tron, Boxleitner's Tron is only supporting, while Bridges' character is clearly the lead.

Jamie Foxx plays the lead role of Django in Django Unchained. Christoph Waltz was excellent in his role though!

I think the lead actor would be Jamie Foxx... He's in more scenes and the entire story is about his character.

What do I know? I'm a bear. I suck the heads off of fish.

I would agree. Had he been a woman there would be no doubt that he should be considered in the Best Actress category. But he is male and apparently you can't have two male leads, one of them has to be supporting.
But how Forest Whitaker could be lead in The last King of Scotland is kind of strange then.

No he wasn't the lead actor in Django Unchained. Jamie Foxx was. If you couldn't understand that, then you really need to learn what the terms "main character" and "protagonist" mean.

It still doesn't feel right. Waltz has so much screentime, dialogue and steals practically every scene he's in (like 80% of the movie), he very much feels like a second lead character. Maybe not quite unlike those buddy movies like 'Bad Boys' or whatever...

Of course, if you have to decide between Django and the Doctor, Django clearly is the lead character. It just doesn't feel right to label Waltz' role as a mere "supporting character" when he was so integral to the plot and the movie as a whole.

Very valid point, there are other movies too where title character wasn't the main character like Tron, Rebecca, JFK, Oscar, etc.

Here is a comprehensive list of such movies http://www.imdb.com/list/ls076216869/