I would like to share my two cents
For Cresselia, it has an interesting effect on the metagame, some good, some bad. When I see how Cresselia is handled, it reminds of how Dragon-types are handled in OU; have a Steel-type to absorb a STAB Dragon attack and you should be fine (a broad generality). In this case, the way to 'handle' Cresselia is to have a Dark-type, Escavalier, or something that can hit it super effectively. This is important because of Cresselia's immense bulk, which means that she can easily shrug off hits that aren't SE or STABed, or both. Now how many Pokemon can claim they have a SE STAB attack on Cresseila? Dark-, Bug-, and Ghost-types can claim this. Only 12 Pokemon can claim this, based on only typing, and in RU. This 'method' of handling Cresselia promotes creativity as NU adds much more Pokemon with the aforementioned trait to deal with her, which adds much more flavor to the metagame.
Cresselia, herself, adds much more creativity in the tier, as she can set up weather, screens, gravity, trick room, and general support. Such magnitude of options means that all the different archetypes are viable, whereas before these kinds of teams had much less options. Weather (Rain, Sun) have a bulky inducer similar to Uxie that can set up weather multiple times. Gravity and Trick Room play-styles have a bulky Pokemon that can set up the preferred field condition. The sheer variety that is available makes the metagame more enjoyable to play, as now offense doesn't dominate.
When looking at Cresselia, Uxie comes to mind. Why hasn't all this happened while Uxie's been around? Cresselia is simply much more bulkier, and has reliable recovery with Moonlight, which lends itself to being invincible. Uxie does, however, have Stealth Rock, and Momento over Cresselia, so Uxie isn't completely dead.
Cresselia overall doesn't do much other than spread status around, and could set up Calm Mind but takes too long to pose a threat. This length just asks for luck to happen, so it generally isn't the best route. The offensive nature that is Gen V, Cresselia adds a much needed change of pace, in that it gives the slower teams a chance!