Bruce Willis created an incredibly adorable yet truly heroic character. You love everyone of his taunts, his language, the way he pushes himself to beat the odds, and his view on what's right.
There are only no real negative things I could find, only small stuff. One is that there are a few 80s elements clearly visible in the film that haven't aged well, like the way the coke snorter is played and some of the female hair styles are done. Two, from time to time, Alan Rickman pushes his acting a bit far on the caricature end.
Beyond that, even with age, it's a great action flick.