Sounds to me like your initial opening sequence is missing two heavy hitters. When starting an AoE pull I Judge one target (this also builds a HoPo and starts the CD running), Avenger's Shield (probably our heaviest initial hit), then Hammer of the Righteous as the first mob makes contact, Consecrate, then either Judge again and immediately SotR or Holy Wrath first. By this point I have some Vengeance built up and either drop a Light's Hammer or self-target and use Holy Prism as an AoE. Note that this is a *general* progression more than a hard sequence. If I *really* want to spike initial aggro I'll blow Wings first, being sure to use either the Light's Hammer or the Prism AoE before it expires. Otherwise I try to save my Wings until I'm at peak Vengeance to get the most out of them.
Some points to remember:
Avenger's Shield is a very heavy hitter. It should always be on cooldown or used immediately with Grand Crusader unless you are saving it for a ranged silence or need to dump HoPo before using the GC proc. Which brings me to my next point.
Shield of the Righteous is off the GCD. You should always use it along with another offensive ability if at all possible. SotR + AS is a particularly hard-hitting, high threat gen move. Also get used to targeting through an AoE pull and spreading your SotRs around, it really helps to keep threat if you have DPS not focusing your target.
Holy Wrath hits hard single target, but since it is a meteor effect, gets progressively weaker the more mobs are in the pull. Past three targets or so I only use it as a filler or to tag something that's only peripherally aggroed onto a party member, like an add that's aggroed onto a healer due to HoTs rolling. Always follow up a snagged mob like this with an actual hit.
Consecrate is almost the opposite conceptually from Holy Wrath, it's lousy at picking up unaggroed targets because it has to tick on them, but its damage is constant so it gets more powerful the more mobs are in the pull. Always try to keep it rolling, but not at the expense of Avenger's Shield or your Holy Power generators.
For most groups you can keep pounding a primary target and let your splash hold aggro, but if you *really* want to lock up a group, or if you have a strong DPSer who consistently gets on a different target, keep switching your target and using Judgement and SotR on fresh mobs. I imagine Double Jeopardy would be good for this but haven't tried it. I actually got used to doing this tanking on one of my warriors during Vanilla, since back then Thunderclap used a moderately high amount of Rage and our cleaves were not guaranteed to tag all the targets in an AoE pull - so you had to spread your hits around to control a group.
Barring an incredible gear gap, this technique should keep the group firmly locked onto you - but in a raid setting, be sure to leave your cotank a couple of mobs. Our threat/damage generation is heavily dependent on Vengeance stacks, and without any you'll have a hard time keeping aggro from a DPS who knows what they're doing. But with at least one, preferably 2+ mobs on a tank, it should be close to impossible for them to pry loose.
Edit: I should also address that it may *look* like another tank is just using Swipe/Thunderclap/Blood Boil and stuff sticks to them, but that's not the case. They're stacking abilities and changing targets like we should be, or DPS is peeling off of them like you've both described happening. Retribution's my primary spec, it's instantly apparent if a tank is being lazy and only hitting a couple AoEs instead of trying to actually build a threat lead, because I invariably end up tanking a mob during the pull.
Edited by Camaeris on 12/10/2012 10:48 PM PST