Electronic viewfinders. I've always hated them, but perhaps that's because I owned a couple cameras back in 2005 that had them (Minolta DiMage A200, Panasonic FZ20). Yuck!
However, I'm unable to deny the benefits of recent high quality EVFs, such as that of Sony's A99; live exposure preview, live white balance, live histogram, with good resolution and low lag times. Yes, you can get all the benefits of an EVF on a DSLR with live view, but then you have to take the camera away from your eye.
I still prefer optical viewfinders, but the prospect of EVFs being the future no longer scares me as much. With that being said, I wonder how difficult it would be to implement a hybrid viewfinder in a DSLR, such as the finder on the Fuji X100. After all, the optical finder is one of the main "advantages" of SLRs over mirrorless, which is why I haven't been tempted by Sony's A99.
If the A99 had a hybrid finder, however, my interest would seriously be piqued.