Aside from that it's not a very massive or substantial tier change, but it is something that is necessary for the growth of the metagame.
Let's first talk about OU. Nothing new has entered OU since Genesect, so there haven't been any changes in that regard. On the other hand, though, the top three Pokemon of the tier have changed massively since the last major tier update, and this one is a bit reminiscent of the beginning of Black and White.
Politoed, Ferrothorn and Genesect make up the current top three of the tier, knocking down long-reigning kings Scizor and Dragonite to 5th and 4th place, respectively. This shows evidence of the massive incursion of Rain since the Therian forms and Keldeo were introduced. Genesect is also very viable in Rain (as it is on any type of team), so the momentum of this particular playstyle has picked up quite a bit from the time a few months ago when Sand was its superior. Other than this shift, nothing much really changed in OU except for Tornadus-I proving to be second-best in comparison to his big brother, as well as Mew, Meloetta , and finally Virizion each proving that they're not quite fit enough for this new metagame.
Oh Mew and Meloetta, it was a short stint, but you both came in at the wrong time.
Underused Overused threats become Overused Underused Threats
UU is probably my favorite tier at the moment, and with the introduction of these supreme offensive threats (two of which previously dominated UU for a time), it is definitely going to continue to be a fun metagame. Joining the ranks is Cofagrigus, the previous RUler of the RU tier, as his versatility as both a massive physical tank and as a Trick Room sweeper have proved efficient enough for the upper tiers. I say it's about time.
Each of these five Pokemon will definitely make an impact on the UU tier, but the one that I think suffers the most is Virizion. For being a Fighting type, each of the four other new-UUs stops her dead in her tracks; Tornadus will surely OHKO any Virizion set, being able to outspeed as well; Mew and Meloetta should be able to take a hit and retaliate with a Super Effective STAB Psychic type move, and Cofagrigus will laugh at any Swords Dance variant. Virizion really couldn't have come any later to the UU party.
But aside from their being able to counter and destroy Virizion, each of these Pokemon will surely diminish Fighting types in UU in general. Heracross, Mienshao, and Scrafty are three other potent Fighters that will likely suffer significantly with the new monsters. Heracross had even just claimed the top spot in usage for the past two months. Will he be able to hold his place on the throne, or will Tornadus blow the tier away and swipe it from him? It is definitely possible, but in order to find out for yourself, you're going to have to play!
The Other Tiers...
RU and NU really didn't change much at all in terms of new threats. Cinccino, Amoonguss and Ditto all continue to be very dominant threats in NU (the former two claiming the top spots) and I think that their potency can definitely carry over to the upper tiers as well. Amoonguss is even effective in all tiers, benefiting from its bulk, Regenerator, and Spore. Otherwise, NU was unchanged. Some threats did rise in usage, however, and were bumped up from "PU" to being actual "NU" status, but since PU isn't even a real tier, they've essentially just moved up from a lower spot in NU to a higher spot. Some of these things include Rapidash, Zangoose, Ditto, Eelektross and Cradily, some of which have seen higher usage thanks to their new Dream World abilities. But aside from that, NU remains a fun getaway from the hyper-competitive standard tiers.
In other news, Stoutland has dropped once again to RU now that perma-Sand (namely Hippopotas) is banned from UU. Without permanent Sand, the dog will likely suffer a massive blow, and I wouldn't be surprised if he sees usage in NU in three months' time. He now has Superpower from the Black and White 2 move tutors, but I doubt that will be enough with a 100 base Attack and 80 base Speed stat.
So as always UU has seen the biggest shift, while really nothing else has changed. The Black and White 2 changes are still taking their toll, however, and there's no telling what new Dream World abilities will be released within the next few months. This is definitely an exciting time to be playing Pokemon, so I suggest you all get out there on Pokemon Showdown or Pokemon Online and make an impact!
Lastly, on an unrelated note, Intended to start a new project of Type Analysis in each tier, but I figured I'd postpone its debut until the tiers shifted so that I can have an updated analysis, so expect to see that soon!