what is the hardest in part playing rock guitar?

arpeggios are tough, but actually acquiring your own, signature rhythm and attack is a hard thing to actually establish and get down. Anyone can strike the notes, but the way the notes are tied together in time is what makes folks want to dance, and "feel" your music. Playing slightly behind the beat creates a swing-like feel, similar to jazz bands. Keith Richards is a perfect example. On the song "Monkey Man", that riff is a little late on the 2-beat, complementing Charlie Watts' rhythms in the interest of syncopation.

I think the most difficult part of playing rock guitar would be the dynamics. You need to be able to play the main tune including all the little extras such as chugging, artificial harmonics, hammer-ons and pull-offs, palm muting, alternate plucking and all these little extras that add to the effect of the overall playing. Most of these techniques can be picked up naturally purely from practising over and over again, I have been playing for 13 years now and these dynamics seem to have improved naturally without any effort.