I wonder if people are coming in from WoW, where even after you've chosen a class there is still some wiggle room for what role you will play in a dungeon party. For example, in WoW you could be a Warrior, but there is a tanking spec and a DPS spec. A Priest can potentially be a healer or a DPS. And a Druid can actually be anything. It just depends on what talents you pick later on.
When I first started playing FFXIV, I honestly didn't really understand the difference between Conjurer and Thaumaturge. I figured they were both some flavours of "mage," but not that one was a dedicated healer and one was a dedicated damage dealer. I figured maybe my chosen skills, or cross-class abilities, would further hone my role in a group. But nope. It was a lot more rigid than I realized.
Games really should spell this out for people. I know these concepts are elementary for people who are used to MMOs -- but if a game is going to hinge on certain strongly-enforced mechanics, then it behooves the designers to make sure players can have no doubt about what those mechanics are.