You forgot the <irony> tags, so essential for whoosh-prevention. For example:
<irony>Excellent point. They’re both blondes except for Eliza Dushku. In fact, except for looking nothing alike, Gellar and Dushku could easily be mistaken for twins.</irony>
That is very common knowledge in the acting business and has been a truism since before Whedon was born. The reason usually given is that in comedic acting, the timing has to be absolutely precise for it to work. In general, dramatic acting can be a lot more sloppy and still look good.
I would argue that they could do that because every Buffy character has the same gait, body language, mannerisms and general feel. I know this because almost all of my female friends in high school adopted it, and I couldn’t figure out what the hell was going on until I watched an episode of Buffy with them.
He was pretty good in We Own the Night–not incredible, but he did blow away Joaquin Phoenix’s acting in that movie. That was weird to see. Like fusoya said, though, he was awesome in I Heart Huckabees.
He put out an interesting-seeming movie this year called “The Interview”, which he directed and starred in as a beat journalist who’s miffed at being assigned to interview a B-movie actress during a slow news week. I’ve heard it’s both a pretty good movie and a total departure from his typecast, although I haven’t seen it yet.
I was really impressed by Bill Murray’s work in Lost in Translation, too, although I didn’t much like the movie.
Leonardo DiCaprio blew me away in The Aviator. I never thought of him as a bad actor, per se, but he definitely had a “pretty-boy” image in my head before I actually saw him act. (I’m the one person who was alive in 1998 and didn’t see Titanic. I’m glad that movie has faded out of our collective cultural memory.)
Illuminatiprimus, have you seen Kevin Bacon in The Woodsman? All of the main actors–Kevin Bacon, Mos Def, Benjamin Bratt, Kyra Sedgwick–were stellar. Sedgwick really blew me away, since she’s so awful in The Closer. And this was the movie that made it clear that Mos Def and Benjamin Bratt are serious actors.
Oh, well then. 
The commentary on Serenity was the first place I’d heard of it, so I figured it was his own little director/producer’s trick.
Monster was the first thing I ever saw her in–didn’t know who she was before. When I looked her up afterward I was amazed that she was actually pretty.
I actually came in here to mention Justin Timberlake in both Black Snake Moan and Alpha Dog. I hate his music but he CAN act.
I suppose it seems a bit ridiculous to mention his name now seeing as he’s won 2 Oscars, but I remember watching Tom Hanks in Big and being quite shocked because all I’d seen before that was Splash and The Money Pit.
The Fact that Tommy Solomon can act (Joseph Gordon-Levitt) was a bit of a shock to me when I saw Mysterious Skin (and subsequently The Lookout)
Tom Hanks in Philadelphia blew me away. I still thought of him as “the non-hot guy from Bosom Buddies” at that point.
Keanu Reeves in The Devil’s Advocate. Nailed the part. Good movie, too, except for the ending (why do movies I like keep screwing up the ending?)
Martin Landau. As a kid I knew him from the awful series “Space: 1999”, in which he displayed the emotional range of a tree trunk. Then, many years later, I saw him in Ed Wood and some other things, and found that he could really act.
Hugh Laurie in Jeeves and Wooster. Up to that point I had only known him as House. Although I enjoyed it a lot the really impressive thing was his range.
That was the other one! I was sitting here wracking my brain, because I knew I’d seen him in a couple things where he did a decent job, but all I could remember was The Gift. Yes, he did do a great job in The Devil’s Advocate and, I thought, held up his own against Pacino.
Keanu Reeves actually hits it off in parts of A Scanner Darkly. The first time we see him under the scramble suit as he loses track of his speech in front of the lodge and starts speaking from the heart and making it up as he goes along is very moving. He has other moments, too. I’d say that a good 40-50% of Keanu’s lines were delivered well in that movie, which for him is amazing.
And come on, Keanu rocked in the Bill and Ted movies!
I started taking Mark Wahlberg seriously as an actor after seeing The Corruptor. Great acting by everyone in that movie.
And in River’s Edge. But that and the Bill and Ted movies are right in his wheelhouse.
Oh my GAWD you have to go rent the Black Adder III DVDs RIGHT THIS VERY INSTANT!!!
OMG WHY ARE YOU STILL HERE!!! REEEEEEEEEEENT! NOOOOOOOOOOOOOWWWWWWW!!!
Guess what I have been watching the last few days. 
Myself, I never really saw Leonardo Dicaprio as a serious adult actor, until Blood Diamond, where he really blew me away. Mind you he wasn’t too bad in Man In The Iron Mask, but I still thought he looked like the arrogant kid in Titanic, which, come to think of it, was pretty much exactly what the role demanded, wasn’t it?
S^G
Waaaaaaaaaaaaaaaayyyyyyyyyyyy back in the mists of time, I saw a movie version of The Three Musketeers which featured Raquel Welch in a comedic role.
Damn – she was GOOD! How well I remember thinking as I watched, wow, I never thought she could act so well, do so well with a comic role – which can be harder than a dramatic role.
I’ve been very impressed with Will Ferrell lately, having really liked his dramatic roles in:
Stranger than Fiction (2006)
Winter Passing (2005)
Melinda and Melinda (2004)
Ferrell strikes me as the typical comedian who has a very accesible dark side. That’s usually what drives them to comedy in the first place. Just look at Jim Carey’s background for an example of a tortured past.
Even in his comedic roles, Ferrell unleashes some of the darkness. Ask any alcoholic about his performance in Old School, for example.
Related thread from before I noticed Ferrell: Best acting performance by a comedian
My “DeCaprio Can Act” moment was “The Aviator”.
You’ll want to see Brick, then, which is basically a fantastic film featuring a fine Gordon-Levitt serious performance.
The first time I saw Gary Oldman.
The second time. The third. The twenty-third.
I have to admit that I don’t think too much of Heath Ledger’s abilities as an actor, but it looks as though I am slated to change my opinion of him once The Dark Knight is released. The latest trailer makes me think that he’s gonna steal the show. Kinda makes me feel like Anne Rice with Tom Cruise as now I feel like I have to apologize for all the nasty things I said about him.