Didier,
It is a challenge to put into words the difference in handling but here goes a try.
Side by side and head on the M8 appears to be smaller and slightly lighter. I know that the dimensions aren't that different but that is the perception in all but looking down on the camera where the R-D1 looks thinner (its the stepped angles and cust vesus the flat plate) even though they are the same thickness. The R-D1 also feels thinner in the hand and the small grip on the back makes it easier to hold with one hand, at least at first. Once you get used to the M8 I think it gets easier. I have never noticed any issue with start up time on the R-D1 and I can't say that I have with the M8 either. they both seem to start in about 5 seconds (I wait until the flashing lights go out). At first I thought that changing ISO on the M8 would be harder but given the small font size on the R-D1, I think it is actually easier on the M8. Certainly mor readable without my glasses. Changing WB is also easier on the M8 though I shoot DNG and don't use it. It is pretty bad in auto mode at this time. Since it is writing bigger files it takes longer using the same kind of card Scan Disk Ultra II 2GB. However, since the buffer is larger you can go on shooting for up to 10 shots on the M8. Display of the image after a shot is quicker on the M8 but once you try and zoom it seems slow to zoom and does not appear to have as large a zoom range or number of zoom steps.
Focusing and shooting with the M8 is quicker and it seems less conspicuous even with the red dot. Others have complained about the strap but I really like it and when hanging from a shoulder it feels feather weight yet very solid in your hands. It is definitely nicer to use with a light lens and so far I like the Rokkor 40/2 for its small size, light weight and sharp pictures. It's a little soft at f2 but razor sharp above that. I have a Leica 28/2.8 Elmarit 4th generation and it seems to be as heavy as the camera so I don't keep it on even though its greater DOF would make it more useful on the street. I guess I would summarize the diference in feel like the difference between a Seiko watch (like their high end $6K models) to a Ulysse Nardin (about $15K). One has a feel of luxury and the other feels good but isn't quite there.