The difference is that we allow user contributions that's why our library is a bigger one. We do list those collections that are listed on Postman's official web-site, which are submitted by the API providers themselvves. So that makes our list more complete. Also we have comments and replies on each API page for users to discuss and help each other in solving technical difficulties.

How would the quality of crowd sourced collections be enforced?

I guess the user upvoting system that we implemented similar to Product Hunt or Hacker News would help to keep the quality of the collections high enough. By the way we have built this web-site without code based on a Product Hunt Clone template from here https://zeroqode.com :)