To note: These are first impressions and based on guesswork since we don’t know the exact details.
Quercus will be the third [Buffer / Abjurer] Supporter, the other two being Tsukinogi and Nine-Colored Deer, all three of which have 5-star rarity. All three provide the [Shelter] buff with their talent, but where Quercus is particularly different is that she applies the buff to allies with HP above a certain percentage, rather than below. A major issue with Shelter applying below an HP threshold is that it does nothing to OHKO, and healing an injured Operator causes it to lose the buff. The Quercus talent will be much easier to apply consistently, as it will take effect before the operator takes damage (e.g. on first hits) and will be activated by healing support. On the other hand, her supported allies will become more vulnerable if they take damage and cannot be sustained above the threshold, so high it may be significant.
S1 will make it easier for him to play a permanent healer role, or swap between them. How easily she can trade will depend on the skill’s charge time. S2 seems to be the most interesting skill, as the SP battery utility can be extremely useful, especially for operators in the healing role. The quality of this skill will likely depend on its availability. Another thing to keep in mind is that SP is earned by the Operators it heals, which means they must take damage to gain a heal (and therefore SP). This means that, in most standard situations, SP will primarily be awarded to blocking units and will not be effective for charging skills between enemy waves.