Power creep has been occurring for years, rather than complaining, the best course of action is to adjust to it. Fortunately, I feel that several actions recently have reflected this, such as the creation of more and more tiers. Even NU is a relatively new creation. This has allowed for the creation of more and more superior Pokemon to be created, while also creating a place for the newly-outclassed mons to be used.
This same ideology has been applied w/ the (albeit temporary) unbanning of various mons at the beginning of the generation, like blaziken, the deos, and gene. At the time, they were deemed too powerful for the meta. However, this doesn't mean that they will always be. Some day, the power creep may be great enough that they could come back down again. Probably not, but garchomp proved that it can happen. For this reason, I applaud the OU council for this move.
However, I don't think giratina is the right mon to bring down at the moment. It's just to good. The restriction to a specific item is really not that significant. Especially when you consider that, 1. It mitigates it's otherwise serious knock off weakness, 2. Other mons will carry type boosting items, like Draco plate (dragalge), sharp beak (TF), or black glasses (bisharp), and their respective items only boost one of their STABS, while giratina's boosts both. 120/120/90 attacking stats are no joke, Thats better than Altaria in every regard. Altaria has Pixilate, Gira has griseous orb boosted draco meteor/outrage, which does way more than Altaria could ever dream. Pixilate only provides 1.083x more boost than griseous orb. Altaria gets DD, Gira gets CM. That's only offensively; let's not forget that alt is flying/ dragon pre-mega. This makes it weak to rocks, and after you mega evolve alt, it is susceptible to spikes as well. Gira has neither of these flaws, allowing it to function as a great defogger. Perhaps most important tho, is its ridiculous 150/100/100 bulk. It does lack leftovers, but let's not forget about good ol' rotom w, who, while capable of having leftovers, also relies on painsplit/ rest for recovery. It has 50 HP, and has always been considered one of the premier defensive mons of the tier. It has 1 immunity; Gira has 3.
Neither of these are perfect comparisons. It is soley meant to compare Gira to two of the best mons in the tier in their respective roles. Obviously, this is not a complete analyis of Gira's strengths and weaknesses; that would take forever. The point is, Gira is kinda like aegislash; it's capable of running completely different sets to fulfill multiple roles, much like aegis did with its sub toxic set. Except Gira has WoW.
Gira is just too strong as both an offensive and a defensive threat in OU. It's biggest flaw is its lack of recovery, but that's not an unmanageable problem. While I think it's a super cool mon, it really has no place in OU.