Occulus often starts this way: 5 people zone in. 2-4 of them immediately drop group and port out. Why? Because they've done this before and they know that on average, even if they know what to do, they'll probably die at least once trying to finish the final encounter because someone won't. You see, having to get to know new abilities is a pain, but it's a one-time pain. Once you know what to do, you're all set. The problem is that you run a high probability of getting people who don't know what they're doing. They won't be like the warlock who's inexplicably doing half the dps they should be. They'll fail utterly to execute their role because they don't know what it is.
So, how do you fix that? There are three ways:
- Vehicles have no abilities and role is easy to understand (e.g. you're in a chopper... drive away from the boss until it fades)
- Vehicles have very simple abilities and are used only for a small part of the fight (think DTK final fight, but even slightly simpler and only once per fight).
- Vehicles are an optional element of the fight (e.g. one player must get on a giant and use a complicated set of abilities during a boss fight). This is the Wintergrasp model.