I don't know which one I hate the most between Calendar and Mail.
Calendar~ I use it with an Exchange server and often accept appointments or meeting, but if I restart the application, it seems to forget about it and the event is marked as new. Problem: If I reply to the invitation once again, the creator of the event will receive another email, bad bad bad... I also hate when Calendar complains 10 times in a row that it cannot connect to the platform when I am actually offline, extremely frustrating to have to deal with all these modal dialogs.
Mail~ Not mentioning the poor user experience, the modal dialogs complaint apply as well, mails are not often sent, HTML signature is a pain to setup, often becomes unresponsive, or simply crashes.
All this to say that for a development environment, OSX is my OS of choice, but it is about emails, contacts, meetings and so on, the embedded applications are not good at best.
I had a laugh yesterday when the MacWorld editor said about Apple that they were not only doing good OSes, but excellent applications as well during the TechCrunch talk about the iPhone 5. In my experience, I have always found Apple applications average at best, even on the iPhone.