Lissandra is a champion gated severely by her prevalence in Professional play.
Her current passive is nothing more then an excuse to give her exceedingly high mana costs.
Whilst it's not the only mana passive in the game, with Nunu, Evelynn, Xerath and pre-MYMU Veigar all having Mana-based passives, Lissandra's is the most low-effort and least interactive of the bunch.
Nunu's requires him to adjust his build, Evelynn only benefits from hers when she's invisible and is de-activated upon being visible, She's another culprit the same as Lissandra, that being said she's in line for a VGU already. Xerath is required to interact with his enemy laner to reap full benefits.
Lissandra's Passive on the other hand is only a stationery cooldown, and is only lowered by doing something she would be doing whether she did or did not have her passive. Not too mention it's only her Q and Ultimate that apply to her passive anyways, considering W roots, and E does not slow and Spell effects are not taken into account.
Lissandra is not fine the way she is and Riot has already expressed that.
They aren't satisfied with her current passive and thinks it could use some serious work.