Short of deliberately sabotaging the plot in ASB ways (Darth Jar Jar, etc), it's going to be hard to turn Episode VII into a full blown prequels-style disaster. There's no wayKathleen Kennedy, let alone Disney, is going to let the film turn out that way. That said, it's very doable to have the film turn out to be just okay, or a "eh, whatever", both of which would be seen as a pretty big letdowns.
For instances, keeping Lawrence Kasdan out of the picture and having JJ bring along his writing team from Star Trek (Roberto Orci, Alex Kurtzman, and Damon Lindelof) would hamper the film from the start. That, or have JJ turn it down (Lucasfilm went through a lot of directors before they landed on JJ) and they end up with someone like Colin Trevorrow or Josh Trank.