From what I've seen so far, this whole game revolves around Heatran, and relatedly is heavy on the 3 big S-threats: Scizor, Salamence, and Shaymin.
The thing is that suspect thread was already headed in this direction-- shaymin just took up the spot that Celebi would have filled. The difference between this metagame and the previous, was that all the top offensive threats just have ridiculously good synergy together.
Garchomp, Lucario, Gengar and Gyarados, once the major threats, didn't really have the synergy to just be tossed carelessly on the same team. This is different from what we're seeing now.
Like I said: It's all about Heatran. Heatran is the counter everyone pacts to deal with both Shaymin and Scizor (when you feel you can predict easily). Heatran also has the Ice, Fire, Dragon and Steel resists to support and play an excellent teammate to Salamence, Scizor and Shaymin. Heatran can even be a last fail-safe to stop a salamence sweep (if it's locked into outrage). Here's another important note: Heatran is often the best switch into Heatran. With this, no one can have a team without heatran. >.>
Now let's look at the surrounding pokemon:
Shaymin, Salamence (and the ever present Zapdos/Gyarados) have the ground/fight resist that synergizes them even better with Heatran. Moreover, with Substitute/Dragon Dance respectively, they pose major set-up threats against a Heatran stuck in Earth Power. In Shaymin/Salamence's case (and maybe shaymin especially) they also have the water resist needed to back heatran up, and Shaymin might be the best answer offensive teams have ever got to Bulky Water types (who even now give issues to a team dependant on Salamence/Tyranitar/Scizor (metagross)/Heatran).
Bullet Punch Scizor was going to be a top threat no matter what, and its presence only secures this heatran-central metagame even more. Scizor is not only countered by Heatran, but it is also related to Salamence and Shaymin-- resisting their STAB moves and providing a threat to them if they're weakened (lol STAB-Technician-Bullet Punch).
Salamence/Shaymin/Scizor as the top threats punctuates the necessity of steel types in general, resisting the STAB moves of all 3 and the ice moves most feared by Salamence/Shaymin. The banning of Garchomp, OU's formerly top Earthquake user does not hurt either. (lol, magnezone)
Other pokemon: Bulky Water use is going up even more to deal with the general offensive trend, but now have to watch out for shaymin (quite frankly, I love the bugger if only for this purpose). Levitators/flyers are even more integral in this heatran-centric mentality (now that they can actually defeat the ground using threat who's name is not "garchomp").
Conclusion: Heatran's the man! I don't think I remember OU's top sweepers having as much Synergy as they do now. Salamence/Shaymin/Scizor/Heatran are not only the top threats, they also unfortunately counter each other and fit on the same team really well. Meaning this OU reminds me more of rock-paper-scizors than any before. >.>
Salamence / Shaymin / Scizor / Heatran / Bulky Water (or Ground) / Lead Pokemon (azelf, deoxys, gengar, tyranitar zapdos)
there! team done! lol
About Shaymin though: The metagame was going this way even before Shaymin hit the stage. Heatran was bound to become king of the metagame not only because of its own incredible abilities, but synergy the same relationships with Scizor/Salamence and Celebi were bound to create this same mentality. Shaymin only out an extra milimeter on the exclamation point in my opinion.