As many people have pointed out, Soraka's main problem isn't a power thing, it's mostly the fact that when Soraka is played optimally, the results are kind of boring at best and frustrating/pointless at worst.
There's a lot of fun Soraka playstyles that are varying in viability - Tanky Soraka with MR shred, Jungling Soraka with anti-ganks and aggressive denial Soraka. However, their power levels don't really compare to pure support Soraka at the moment.
I know people are taking a look at her - but it's a tricky puzzle to solve. On the one hand, she is our most straightforward healbot support - she undoes what the enemy does. On the other hand, how do you make that an enjoyable experience to play as / play against at all? How do you encourage off-builds to be more effective when someone playing Soraka sees the opportunity to take a more prominent role after crushing their lane?
Personally, I strongly believe this is kind of an issue with level-up incentives: Compare rank 1 'W' to rank 5 'W' and see the 10x jump in effective health increase that the skill provides - This kind of makes every other skill incredibly unappealing to level, mostly because the power disparity between a Soraka with Rank 2 'W' to rank 5 'W' can be 5x, while the jump in effectiveness of her other skills don't scale anywhere near that high...