Every description of Elemental Focus I've come across so far says it reduces the mana cost of your next 2 damage or healing spells.
EDIT: said descriptions being Yarr! and
http://urlshort.com/wotlk/site/talentlist.php
One shaman chain-casting spirit link and using no heals is worth his raid slot iff the redistribution of damage allows an extra healer's-worth of healing from the other healers. I would consider this to be true on a 25-man tank&spank fight since you either double or triple the main-tank-healing that can be done with prayer of mending, renew, bubble, chain heal, and Tree Form druids in general. If the other healers in your raid are all paladins and holy priests, you don't get that much benefit out of it, if you're stacked with the two resto specs it's godly because it converts suboptimal healing scenarios into optimal ones.
Spirit Link is also the closest thing you guys get to an instant heal. It's useful preventing gibs from RSTS casted (or otherwise predictable) nukes. And it while it will take experience to confirm this, I suspect its PvP applications are situationally really nice for larger-group arenas because you effectively put a few thousand life on the assist-train target. You soften up the rest of the team for AoE and such, so it's not completely overpowered or uncounterable, but it changes the dynamics.