I don't believe that most users will put up with poor design.

It's just that some sites like myspace have so much momentum that users will go to them despite their poor design because all their friends are there already.

The problem in terms of design is just the opposite. I would say that users' high expectations with respect to presentation have hurt web startups in a big way since many founders can't afford a professional web designer. Flash over substance is always a bad thing.