PIM = Personal Information Management. Calendar, Contacts, Mail.
Sync = Keeping PIM information current and consistent on 3 devices: Laptop, Mobile, Web.
The problem = good luck getting this to work without locking into one platform. And even then, good luck. Case study:
I have a Mac. I run Mail.app, not Entourage, because Entourage is garbage. If I used Entourage, some of these problems would go away, but others would be introduced. There is no open standard or protocol for interoperability.
Contacts. This is the core of PIM. Your mail program must read from your contact list to do simple things like lookup email addresses. Most mail programs deal with this by building in a contact management tool. If you import all your contacts into their tool, then their mail program will be able to look up addresses without you needing to copy and paste between two tools.
I keep my contacts in Google. It's the most comprehensive and flexible contact manager I've ever seen. It also integrates, imports, and exports with many third party contact managers. Unfortunately, "other" contact managers aren't as flexible. Perhaps they aren't as flexible in terms of how many email addresses you can attach to a contact, or are limited in some other way. So, I can deal with importing and exporting. I'd rather not do it - I'd rather have live sync. But I can live with import/export. Assuming I can live with the limitations of certain contact managers, the next challenge comes with sync. When I add a contact on one device, I need it to sync to the other devices. Preferably without having to connect a cable and press a "sync" button.