Some people can listen to instructions on Vent and get it, others are kinesthetic and the fight won’t click in their heads until they’ve been in it. Thanks to youtube, visual learners can watch a boss fight before they have to try it themselves. I love diagrams but sadly they’re not so commonly used in strategy guides nowadays.
Raid leaders should remain aware of the way people learn differently, and bear it in mind when you judge their performance after the first pull. Possibly, some couldn’t really grasp what you said on Vent until they smashed their face against the boss.
Vent’s one-way sound makes it hard to hear an important interjection, and led to the importance of raiders knowing when and when not to talk. I expect a guild who were all sitting in the same room would benefit from being able to hear both their raid leader and the others they were sitting next to, and I bet a PvP arena team would boost enormously from that.
Here’s an article on raid leadership, learning styles and how to give instructions.