Conclusion
Peasants make terrific nuisance encounters: better even than goblins, kobolds or other low-level monsters. The simple reason for this is that as soon as the PCs exercise even a few iotae of brain power on the problem of angry workers they realise that the problem can't simply be solved with a swift and brutal application of remedial swordage. If you slaughter peasants, you are guilty of murder or, if the land is a little less progressive, unlawful destruction of livestock — which is probably a hanging offense as well.
If the PCs insist on walking all over the peasants, have their resentment build up first, get them to go through the correct channels, informing their landlord about the problem: after all, why pay their land rents if some brigands are just going to trample their crops and beat them up? They know their rights! Serfs might not be educated, but they're crafty enough in their own way, and know when to save all that frustration up for when it'll do the most damage.
Don't underestimate the ability of peasants to co-operate with and conspire against each other over the most petty of matters. Small things can easily be blown up out of all proportion. Look at the cattle raid in the Táin Bó Cúailnge, which pretty much amounted to a shitload of deaths over, and let's not mince words here, a bull.
One final note: like so many other things in life though, don't overdo it. If every single peasant the characters encounter is surly and just itching for a reason to take a dump on the heroes' lives, then sooner or later the PCs are just going to opt for villainy, or worse yet, do all they can to derail your campaign and give you a minor mental breakdown. Don't think they won't! You know what vicious bastards they can be if you give them an inch.