I've only been out school for a little bit, but here's my take on the situation:
With shorts you're able to get people to work for free or far less than they would on a feature. At the end of the day, a short film is just a weekend, maybe two. Assuming it works with my schedule, I can do actual work during the week and it doesn't really interfere with my life.
However, features are totally different. They take up a substantial amount of time during which the cast and crew would normally need to be working to sustain themselves. Additionally, peoples' willingness to help out starts to run dry. I once worked on a 40 minute thesis film where, by the end, the location owner, who let us use the location for free, was getting a bit restless. There's no way he would have been able to stand a feature.
I guess what I'm saying is that you can rely on favors for shorts, but not features. This makes features, which cost more because you're shooting more days anyways, exponentially more expensive than shorts.
In terms of finding talented actors and crew that won't bail, I always try to develop a relationship and understand why they want to work on my short for little to no money. I'm much more likely to cast a student who has actually read and is excited about my script than a SAG actor that's sick of being in background parts and student films, but can't find anything better. The one thing I wouldn't compromise on is sound. You need to pay.