A must have for serious tankadins include:
PallyPower - easy buff management and co-ordination, works better if all pallies in the group have it
Tankadin - provides quick and easy access to vital information, including avoidance stats, spell damage, the number of times AD saved your butt, and flashes for when RF runs out. Also, while the announcements are a bit annoying (esp. when you have low hit rating and AS misses a lot), they can warn your raid to lay off a bit on the dps as you couldn't frontload as much aggro as normal
Omen - the bread and butter mod of any tank, and you are strongly advised to force the rest of your raid to install the mod too for the most accurate results
Deadly Boss Mods - not really tankadin specific, but useful to warn you when a boss is about to do something nasty

(think Aran's Flame Wreath

)
Hope this helps
P.S. As for Holy shield...the general rule is to spam it as soon as its cooled down, so no need for a mod for that

And if you find yourself going OOM often, just downrank it. Same uncrushability, less mana
