As a new member of a large raid group, I'm starting to see endgame content I've been reading about for months, and it's an exciting time for me. However, this is the point at which leadership is crucial in order to turn players like me (full of enthusiasm but lacking in experience) into fully-fledged raiders. But how? The following observations may seem obvious, but I've seen leaders who ignore every single thing below. My main suggestion for raid leaders, class leaders, healer leaders and others, when welcoming a new person into your raid: help them feel at home. A raider who feels confused, out of place and unhappy is far less productive than one who has been welcomed and brought up to speed by a helper. Sure, everything they need to know may be on the forums, but raid forums can be huge and confusing -- how does a new person know where to begin?
During fights, if you know the new member hasn't been to the instance before, why not help them find their way through and give them a little background and warning for the fight? Simply asking someone else of the same class to help them takes the load off your shoulders, and stops them wandering around aimlessly. The new member may be proactive and whisper someone for help, but with thirty-nine new names, it's hard to know who will be helpful and who will resent the intrusion.
Finally, the most important way to help new members -- from my point of view as a new member, of course -- is to make sure they're happy, and to listen to them. If they're performing a particular role but they're not enjoying it, and you're ignoring their feedback, then you've less chance they'll come on the next raid. Plus, of course, every second they spend moaning in guild chat about how bored they are is a second they're not paying attention to your raid.
Some raid leaders and groups manage to make people feel welcome and happy straight away, others take longer (or never manage it). But as a rank-and-file peon, I know where my signups are going -- straight to the group that helps me and makes me laugh, and grudgingly to the group that makes me feel out of place. Being part of two distinct groups like this is an experiment, and one which looks to be nearing its conclusion faster than I expected.