As Robert Altman said, the death of an old man is not a tragedy; not so with a young man just at the beginning of what seemed to be a brilliant career.
I had been aware of Heath Ledger as an actor for a long time in mediocre movies like 10 Things I Hate About You and The Patriot before I realized what he was capable of with his strong performance in Monster's Ball, and then even more with his amazing performance as Ennis Del Mar in Brokeback Mountain, which I would agree is one of the finest film performances in this decade.
What his performances had in common was an extreme commitment to the focussed internal lives of his characters, almost to a fault (as with his completely committed, over-the-top performance in Terry Gilliam's Brothers Grimm) His performance as Ennis is quiet, withdrawn, repressed, but Ledger's commitment to his character's thoughts and feelings is so total that we never have a doubt as to what's going on inside of him, the struggles raging within this quiet, unhappy man. It's a terrific piece of work that will, sadly, be Ledger's legacy.
Best wishes to his friends and family.
PS: I don't think the incessant TV coverage was as obnoxious as when something happens to Paris Hilton or Britney Spears, but there was at least one moment of perfect crystallized awfulness that I watched earlier, on the TV Guide network, when one of their people made the segue, "He plays the Joker in next year's Batman sequel, but there was no laughing today..." Good lord.