Steve
Mostly Harmless
It's a classic because it's a movie that a lot of people have seen and enjoyed, and it holds up pretty well. It's a simple plot that invokes all the classic archetypes. It has great, over the top characters, good music, and some highly quotable, memorable one-liners.Why do people think the original is a "classic?" Because it is old? It wasn't a great movie then and I see no reason why they are remaking a decidedly mediocre 80s action flick.
It's also left a mark on pop culture over a period of 40 years. Even people who haven't seen it are aware of it, know what it's basically about, and can probably recognize some of the lines and references.
Not for everyone, but I consider it a classic.
Now, a real cult classic is the lesser known Patrick Swayze movie, Steel Dawn. A true masterpiece of action schlock movies.