I absolutely do not believe Cresselia deserves to be quick-banned. It is very bulky but it is lacking in attacking power and it's threat comes only after it has managed to get a couple of CMs. It suffers severely from 4 move syndrome, it will always have to sacrifice coverage for status protection or healing and its low initial power makes it easily phazed. It has got a bad STAB type that suffers less than perfect coverage.
Its main threat for me comes from its defensive reliability, to be able to almost guarantee screens is a huge factor in a battle. However, even in this regard it suffers compared to Uxie from lack of U-turn (to get a recipient in safely) and Healing Wish is a double-edged sword. Their bulk and speed for all intents and purposes is very similar so the only real advantage Cresselia has is access to Moonlight.
Cress is a dangerous threat if a team comes unprepared, the same situation for any big threat such as Honchkrow, Gallade, Moltres... Its low initial power, severe lack of coverage and 4 move syndrome mean that no matter what variant you face, it will always have at least one weakness. Usually it is a pretty generic, common weakness too: Phazing, Toxic, resisting its STAB and coverage (which is surprisingly easy), Taunt + stall, Perish Song, Unaware or just good old setting up alongside it - any well organised team will have at least a couple of these strategies to deal with it and keep the flow of the battle in their favour. Getting more than 100% in a damage calculator is not the only solution in Pokemon!