When Young Guns came out (1988), westerns were a dead genre. I don't think there had been a decent one in a decade previous. So everyone went "OMG this rocks!".
Then Tombstone(1993), Unforgiven(1992), Wyatt Earp(1994), Dances With Wolves (1990), Maverick were released in a 6 year period after that movie. People said, "Wow, thats a fucking western. Young Guns is a pile of fluff crap." Then the release of Young Guns 2 just reaffirmed that.
But there is alot of good in Young Guns 1. The whole "origin" sets up pretty nice. Keifer Sutherland is awesome as usual, and Estevez probably has his best role as crazy/funny Billy the Kid. The ending was a nice touch also. That being said ... the other movies I mentioned all take a big shit on that movie.