From Bang the Keys author Jill Dearman's interview with John McCaffrey at the Barnes & Noble site - found via Jill's site bangthekeys.com:
JD: How is writing a novel different from writing a short story?
JM: I also work in the mental health field, and I often think that writing a novel is like being in long-term therapy, where you lay on the couch twice a week and sob about the time your mother made you wear a bib to school (not that that happened to me). Short story writing, on the other hand, is like being in crisis counseling – you just need a few sessions to get through a tough period. I’ll spin another analogy - I feel you must approach the writing of a novel with the determination of a marathon runner, prepare for it with diligence, and then once the gun sounds fight to the finish line. A short story is more a sprint, and a good sprinter runs on nerve just as much as legs. I also think that unlike a short story you can meander in a novel, take more sidetracks, since the reader has made a commitment to a longer work, and thus will conceivable go with you in your fictional travels – as long as you invite them nicely.