I can understand why some people would dislike mean streets due to it's lack of plot and structure should they have watched the movie only once. It is, however, the same as passing comment on good music after only listening to it once. Impossible to judge, in my opinion.
Scorsese plays heavily on his childhood in content, introducing the audience to his world through the eyes of four local hoods. There is none of the morals of it's contemporary mafia based film, The Godfather... and none of the thrills and wealth portrayed later by Scorsese in 'Goodfellas'. It is a real world where gun crime is unusual and shocking and violence is sporadic and adrenalin fueled.
The cogs that keep the film moving forward are that of Charlie's questionable faith and his desire to prove himself by helping Johnny Boy free himself from a mountain of debt he has built up with Michael, a small time shark. The centre point for the scenario is a bar owned by Tony, and the four players weave in and out of each others lives with tensions getting more serious and a downfall becoming more inevitable as the film progresses.
Mean Streets is also improvisational comedy at it's best in parts. The relationship between Charlie and Johnny Boy (and the sheer talent of the two leads) allow much unscripted conversation to flow and it leaves you grinning widely, if not full out laughing.
I believe that taste is accountable for most things, and quality comes to a slightly lesser extent. To me, this film has something that I cannot put my finger on that makes it shine brightly. As mentioned before, it demands multiple viewings, but give it a chance... and watch it on the big screen if you're lucky enough to have it shown locally, and you might well discover a film that takes pride of place as your favourite, just as I did.