Cynics are people who have a "no" posture. They begin trying to figure out why they're against what you're proposing before a conversation has begun. They know they're against whatever it is you're doing - they just haven't figured out why yet. Cynics often feels as though they're the most important person in the organization. It's their self-given role to keep the leader "humble," or "in line." They're not interested in moving forward; they're interested in being right at someone else's expense.
It's in the organization's best interest to run cynics off (or marginalize them) as quickly as possible. They don't add value, only division; chipping away at the foundation of the organization one objection at a time.
Skeptics are late adopters. They take a long time to warm-up to ideas and are often difficult to convince. They often give a lot of push-back, but for very different reasons. Skeptics work from a "caution first" posture. They want to be convinced, they're just not yet. But if they ever are convinced, they'll become the biggest champion for your idea.
It's in the leader's best interest to have at least one or two trusted advisors who are skeptics. Unlike the cynic, the skeptic is for the leader and the organization. They can keep a leader from running too fast, too far, or in the wrong direction. They often (not always) will shine light on the leader's blind-spot and help him consider perspectives he might not see on his own. If nothing else, they will help the leader shape communication to take various perspectives into account.
The challenge for leaders is this: it's far too easy to mistake skeptics for cynics.
When a leader is passionate about something, opposition or criticism of any kind is hard to take. Too often, when we receive push-back - especially strong push-back - we immediately assume the source of the criticism is a cynic and seek ways to marginalize him. This kind of response only betrays our arrogance and often prevents us from hearing feedback that could help us lead better.
Be on the lookout for cynics and root them out. They'll kill your organization. But beware of giving someone the "Cynic Tag" too soon. You might marginalize someone who could have helped you go further, faster, while accumulating a lot fewer scars in the process.