I think the only thing that really irritates me is a set, confirmed, specific-time appointment that becomes a NCNS.
Expressing interest broadly or a complement and then disappearing [completely or for a while] is absolutely no issue. Expressing interest in a given date [as in, I'll be in town on x day looking for a 2 hour, would you be available?] and then flaking is a bit disappointing but no biggie. Canceling up to like, 24 hours beforehand without an excuse is no problem at all. Canceling (so long as I haven't booked a room or declined another offer) up to 30 min before with an excuse is also understandable.