I just realized all the buffs Venusaur gets under the sun. Double Synthesis recovery, Growth's boost doubles it's attack and Sp. attack (an SD and a NP boost in one go) and it's speed is also doubled with Chlorophyll. Damn, this thing is a beast under direct sunlight.
Not quite as effective outside of it though. Sleep Powder and Growth are still good, but Synthesis gets nerfed by other weather conditions and Chlorophyll is useless outside of sunlight.
Still, Venusaur has access to just about all of the worthwile Grass moves in the game (except Leaf Blade and Seed Flare, the latter of which is a Shaymin exclusive so it shouldn't count.) It gets Energy Ball, Seed Bomb, Grass Knot, Solarbeam, Leaf Storm, Petal Dance and Power Whip. I don't think there's a single other Pokemon out there (besides Smeargle) that has access to all of those moves at once.
It also gets fairly okay coverage (EQ, Sludge Bomb, HP, Outrage) and Sleep Powder (and Leech Seed) for utility. This is his biggest drawback. It's list of coverage moves does leave a lot to be desired. Still, with SD and Growth, all's not lost. Thanks to it's decent bulk, it can boost it's stats to the point were the few coverage moves it does get become deadly enough.
Outside of the sun, Venusaur still makes a pretty damn solid UU Poke, but under direct sunlight? Venusaur is a freaking monster. If Drought Ninetales becomes popular enough, he might just follow her into the OU tier.
He'd make a fantastic mixed sweeper with Growth, Physical sweeper with SD (or Growth) and Special sweeper with Growth (under the sun, you can even afford to use Growth for just one of it's stat boosts, ignoring the other, since it's essentially SD and NP rolled into one) and tank (thank you Synthesis). If only he had better coverage. Still EQ and HP are excellent ways to round out Venus weaknesses (especially with Growth under the sun and Chlorophyll).
Without perma sun though, I see Venu sticking to UU.
Maybe he'll be banished to BL? Sun sweeping may just gain a ton of more appeal in the lower tiers thanks to this guy, Solar Power Charizard, Chlorophyll Leafeon and Leaf Guard Meganium (even without Drought Vulpix/Ninetales) that he might still be considered broken in UU.