Using a healing class, having this information on-tap is an absolute necessity.
Particularly being able to configure it somewhat (ie. to display amount of life missing instead of just a current HP value).
Currently the only real option I've seen for achieving this is an addon called SP_Frame, which is really outdated, and it only 'sometimes' feels like working - yeah more than 80% of the time it errors out and won't display anything. And even then, unless you get your hands dirty with the lua file, you won't be able to select any sort of deficit figures.
For the player frame, I've always used HealthValues, which is absolutely fantastic (easy to read font which hovers over the bar, but only displays when there's actually health/mana missing on each bar or unless mouse-over).
Something that would perhaps take over these 'party values', give them the option to display over the bars or to the side of the bars, and have configurable text values (and reliable code that would work most the time to boot!) would be an absolute necessity for anyone that only wants to lightly modify the default unit frames.
If anyone could take this up, it would be greatly appreciated :)