Y'know this chapter got me thinking that a lot of stuff that has been happening since the beginning of the series was pretty much setting up this event. I do apologize if this has been pointed out before but I'm going to illustrate a few points:
1. Almost EVERYBODY sans Luffy (up to this point) and Sanji in the Strawhat crew had a tragic death push them to become who they are in the present. I always felt Luffy got the least tragic flashback sense Shanks is still alive. THIS is Luffy's tragedy.
2. Oda has always cheated death with major characters in the present time but always displayed it in the past. Up until now it kept the manga seem not as dark, I think Ace's death is Oda saying "nothing will be the same from here on out."
3. We've had several arcs where despite the odds Luffy always achieved his goals but behind losing his crew this is basically Luffy's "break the cutie" moment. I'm glad he's actually suffering because it will actually make him a stronger pirate king in the end.
As far as Luffy's expression I think it's perfect. It's an expression of utter horror, we've NEVER seen Luffy in any true grief (losing Usopp it was very restrained, losing his crew was frustrating and humiliating) because the other incidents at least there was some degree of repair. Ace, OTOH, is gone. The expression looks like either two things will happen next chapter:
1. Luffy will pull a SSJ2 moment with his "haki"
2. Luffy will be catatonic causing the timeskip to occur.
In the end I gotta say Oda congrats, you've taken something that Naruto tried to do and make it better (killing off a "certain" brother) and I really can't wait to see this animated just to see if Toei pulls out all the stops like they did for the Going Merry, Bink's Sake, and Strawhat Separation. When this happens in the anime it will be truly epic!
O-chan