I've mainly been running as GLD, but I've also done a couple runs as MRD. Threat has been a nonissue for me. Stick to you standard rotation, every 1-2 cycles, toss a flash as needed to keep extras from peeling off to the healer. Like others have said, stay ahead of the dps.

#1 and really the only rule that matters: Situational Awareness. This applies to every class in all MMOs, but especially to tanks. Don't get tunnel vision. Keep an eye on the corners, watch for patrols, keep your mobs facing away from the group (especially with cleave attacks), watch for stun timers, rotate your cooldowns, etc. Don't be afraid to be in charge. Most groups expect you to be the leader by default anyways. You're the one pulling trash and leading the way. Read up on the dungeons beforehand if the info is available. If no one else in the group knows what is going on, you need to be the one to tell them, because you'll be the first one eating it if the others don't know the fights.