Quoting Red Alert in OU October Statistics Thread:
This is actually an important question to answer when we're banning mons. I hardly view Deoxys-S as a mon that is stifling viability of Pokemon in the OU metagame. It's no Excadrill, which considerably restricted team-building. It's not even Thundurus, which allowed for more flexibility in the way we deal with it, but still a very dangerous threat with its troll Speed, Nasty Plot, and awesome coverage (Focus Blast).
I never found myself being restricted in my team-building process, due to the presence of Deoxys-S. Even when I face one, the LO variants are easily revenged with all the priorities available to us and 105+ Scarfers (Gengar, Latios, Terrakion, Mienshao, Chloro Venusaur). Can't say the same for Excadrill, which resisted common priority moves and was not checked by Scarfers below 120.
The lead sets are annoying, but they are in the same league as Ferrothorn or Dual Screen Espeon, as far as I'm concerned. I honestly have nothing much to say about them.
I fail to see the removal of Deoxys-S affecting the metagame diversity, as it had in previous bans. Dual Screen + 5 Sweepers will still thrive by simply replacing Deoxys-S with Espeon / Azelf / Uxie / Latios / Jirachi. Ferrothorn can easily set up Spikes / SR, thanks to its defense and typing (Yes, Deoxys-S has the Speed, but priority moves can easily limit it to laying only SR, a feat most mon can easily do without dying), while not totally forfeiting the offensive momentum thanks to Thunder Wave / Leech Seed.
We simply banned the "best thing" in OU, and I guess we move to the "next best thing," to ban and lower the overall power level of OU?
I wouldn't be surprised if Dragonite / Volcarona gets the banhammer after Deoxys-S goes, b/c "setting up Rocks isn't as easy anymore," lol :/