In 4.x, every role has some form of enmity management under the shared-role actions. That's because if you're trying to play optimally as a team, then it's everyone's responsibility.

That being said, if a dps decides to never use those tools, you're still going to find a way to hold aggro. But that's not because it's your sole responsibility, but rather because skilled support players will always try to find a way to work around underperforming teammates to get the team the clear. If you're in a situation which requires a high level of performance, though, there's generally no room for slack.

Tanking cannot be neatly defined, because you will always expand out to fill your team's needs.