I would say..
1. Organizing the argument (i.e. choosing what is relevant and what is not relevant). I've heard people get the angle completely wrong...and even if it's presented brilliantly.. the message they have chosen will still be wrong.
2. If they do know the message... getting that message conveyed simply. people try to make themselves look intelligent and overcomplicate everything. the message gets lost.
3. Presentation skills... from powerpoint to presence.
The last can be taught in training but the first two I'm afraid require a bit of inherent skill. They may be taught if trainers were actually critical with presentations... every training event I've been to has been 'great' ..'fantastic' even when the product was crap.
Hope that helps.