Interesting question.
The Luz-Ardiden stage has a nice succesion of mountains, but Luz-Ardiden itself isn't the most difficult mountain (although it's not easy either). Reminds me a bit of the stage to Hautacam in 2008. If someone is willing to use his team on the Tourmalet, 1 or 2 'favorites' could be eliminated already in this stage.
Plateau de Beille is probably the hardest mountain they finish on, but the rest of the stage isn't that special, too much space between the mountains. For the GC-men this is going to be an 'easy' stage untill the final mountain.
Now, the Galibier. Certainly the most difficult to predict stage. Barely above 5% until the last 5 kilometers. But you have to take into consideration the great height they are riding on (finish at 2645 meter), and the fact that they were well above 2000 meters twice in the stage already (and on really difficult mountains). Will those factors be enough to offset the low gradients, or do we have to wait untill the last couple of kilometers in this stage to see any action (and small differences therefore)? I really can't tell, but with the Tour, it's usually the best not to expect too much.
The Alpe is about as difficult a mountain as you'll find in the Tour. The great thing about this stage is that after only a couple of kilometers, you'll immediately get the first mountain. There will be some crazy racing on the Telegraphe to get into the break, and if the wrong people are in a break, you could have some teams setting up pace all the way to the top of the Galibier. I think the first 50km of this stage are going to be hell. Although the comparison ends there, the beginning of this stage reminds a bit of the stage to Pau last year (were Armstrong attacked), and some GC guys like Gesink and Sanchez had a really difficult time on the Aspin there, a lot of climbing early on will always hurt some guys.
After the Galibier it's a long easy descend to the Alpe, a lot of guys can come back in this part of the race. What's good about the Alpe D'Huez is that it got some really steep parts at the bottom, which usually means that all but a few domestiques are eliminated almost immediately. The last 10km is every man for himself, I think that combined with the hellish start of this stage, is going to make this the most difficult MTF of this Tour.