One thing I think is important for a healer UI is to let you see important buffs. In general I think displaying relevant information is one of the essential features of AddOns, but in particular I think a healer benefits from having an accurate view of the raid's status. This view naturally would include debuffs, as we often need to dispel them or heal people affected by them. But I feel another important part of that view is the display of certain types of buffs:
1. HoTs: Naturally you want to see who has your HoTs active. You may also want to see when people are covered by HoTs, so you know there is some healing coverage there. GridStatusHots provides a great display of these.
2. Shields: Disc priests will want to see who has PW:Shield active, but all healers can benefit from this: seeing a shield lets you know there's less need for immediate heals. An active shield is like a few thousand extra temporary hit points, so you want to see shields to get a proper view of everyone's health.
3. Immunities: Spells like Iceblock and paladin bubbles can be good to see, so that there is no need to focus healing on a target that won't need it.
4. Mitigation buffs: It's very helpful to see when a tank pops Shield Wall, Icebound Fortitude, Last Stand, or a similar skill. It shows that the tank will be taking less damage, and it indirectly conveys info on what kind of damage the tank expects in the near future, and on the condition of the encounter.
5. Lesser mitigation buffs: Things like Shield Wall greatly affect healing, but there are also lesser mitigation effects. Grace, Inspiration/Ancestral Fortitude, and Blessing of Sanctuary all can reduce damage by a few percent: not really vital to know about, since their presence probably won't change your healing decisions, but nice to see. Divine Aegis may fit in here too, as a typically smaller shield spell.
6. Reactive healing buffs: Prayer of Mending and Earth Shield should definitely be tracked by priests and shamans. MendWatch and other mods track these for the purposes of knowing when to recast them, but it's also good to have these display on your unit frames, so you know who can safely take a hit from an add or AOE without losing HP right away.
7. Back from the Dead spells: Spirit of Redemption and Shadow of Death allow players to keep playing even after they die. Most unit frames will show these players as still alive and kicking, even though they are living on "borrowed time." Naturally you want to know which raid members won't be with you much longer!
Other buffs can also be useful to see, from boosters like Arcane Power, to regen spells like Innervate, to encounter-specific spells. They range in important from vital to know, to mere convenience. But I think overall, any healer can benefit from thinking about how seeing some of these buffs will help them.