Honestly, this is the problem I have with tanks in general. If you are new to MMOs, DO NOT and I repeat DO NOT roll a Tank. Just don't do it. I'm so sick of bad tanks tunneling on a single target in a pull of 5-6 monsters and thinking they are some gift of the tanking gods while the rest of the raid does HIS/HER job for them. This is purely coming from a Healing PoV. And if you think your job as a tank is to eat damage and die, re-roll a toon now before you make your server mad at you. The role of tank is never to die (though you do eat some damage, but mitigate it when possible), it's to keep the rest of the raid from taking unnecessary damage.

If you have an issue holding aggro, then mark targets as mentioned. If they pull off you on the target you are focusing it's YOUR fault no theirs. If they pull a non-focus target, let them tank it, have the healer understand that they are not to get heals and watch the fireworks begin when they die from their stupidity.