Every now and then (or abt 1-2 times a year), I audition electric guitarists for my church's youth music ministry. Some focus too much on playing the lead parts without basic rhythm foundations. Common mistakes include getting the tempo wrong, using too much distortion, and the ever common 'rock star' syndrome where the guitarist solos too much. Many guitarists also neglect getting good clean parts; another common mistake is to ignore the dynamics of the song.
I recall an auditionee who couldn't play the lead parts for the songs, but was awarded a higher score as his rhythm foundations were great. Hence, get your rhythm basics right before attempting the lead parts; always choose good rhythm parts of bad lead ones!
Here's a rather exaggerated compilation of common lead (and some rhythm) mistakes - have a laugh, but watch out for your own playing!