Billie Joe Armstrong started mixing his vocals with some weird reverb screen that makes his voice sound much cheaper in 21st Century Breakdown. I know it sounds stupid, but literally the ONE THING that makes me not enjoy the album is this reverb thing.
Without the effect in American Idiot, his voice sounded nuanced and clean. If only he hadn't mixed his vocals with reverb, I'd probably rate the album 4/5 rather than 3/5. It is THAT annoying.