That's the thing - you can't just "tag" mobs because the aggro mod is 150% (compare that to the absolutely HILARIOUS 500% in WoW... 300% was fine, but 500% is just ridiculous).
What I've found out through talking to people here and also tanking a FP or two is that you only ever have to tank elites (gold star or up). It's simple - normal and silver enemies will die so horribly quickly and output pitiful damage that it's just not worth it to spend time trying to tank them.
What I do now is mark the elites for kill order, instruct my group to ALWAYS kill normals, then silver, THEN switch to elites, in the specific marked kill order. With proper players and teamwork, this works wonderfully. Also, obviously, *always* Guard the healer - healing aggro is at 50% of amount healed (although I don't know if overhealing causes aggro) and with guard it drops down to pitiful levels.
I will say, though, that Guardians, while being very hard to kill in PvP and in FPs, have absolutely horrible AOE abilities (Sweep is good but the CD is WAAAAAAY too long, not to mention it's the only way to apply that specific debuff, so if adds come into the fight, well, too bad, AoE taunt comes in way late and the AoE slash is weird because it targets 5 random enemies if there are more than 5 mobs) so it makes tanking trash a bit more annoying. As an aside, with my Vanguard, at 25 now I have 3 AoE abilities (2 of which are absolutely insane) - Sticky Grenade which does very high damage (very useful when there's 2-3 elites as it builds aggro on multiple mobs very quickly and is also very high single-target dps), Pulse Cannon that does an absurd amount of damage (but can be interrupted) and Explosive Surge, which applies the damage debuff while doing AoE with NO COOLDOWN. Add to that a much simpler priority rotation (High Impact Bolt > StockStrike > Sticky Grenade > Ion Pulse = Explosive Surge > Hammer Shot = Energy Blast if low on Ammo) and it makes tanking much easier. Sure, their survivability might be a tiny bit lower, but in general it's offset by making tanking much easier.
I liked my Guardian, but god my Trooper is so much more fun to play with.