Well to start things off, the prospect of Drizzle and Drought in OU gives Roserade all that much more potential to use its Weather Ball.
As usual, I will be focusing on what sets Roserade can use in Gen 5, as opposed to listing every single set including those unchanged from Gen 4... It definitely improves from Giga Drain's power boost. Solarbeam and Synthesis are all considerable if Roserade is partnered with Ninetales.
Special Attacking @ Life Orb/Choice Specs/Choice Scarf
252 SpAtk, 252 Speed, 4 HP
Sludge Bomb/Venoshock/Extrasensory/Weather Ball
Giga Drain/Grass Knot/Magical Leaf/Petal Dance
Sleep Powder/Hidden Power/Rest
Although Rose's defensive sets are pretty much unchanged, Roserade's Special Sweep set received an interesting change: Technician. If you choose to employ this Ability, Roserade has improvements in access to Magical Leaf packing as much punch as Sludge Bomb and base 90 Hidden Powers. Otherwise...no difference. Roserade has a niche in having the greatest SpAtk for all Grass-Types (tied with Exeggutor) and the greatest SpAtk for all Poison-Types (tied with Gengar).
Venoshock is also an option over Sludge Bomb if you judge your team can spread Poison very well.
Petal Dance received a power boost. Just like how you would use Magical Leaf, Giga Drain, or Grass Knot in conjunction with Leaf Storm, Petal Dance is meant to be used if you do not intend to switch out Roserade the following turn.
Leaf Storm: Roserade's big gun. Hit hard, switch out.
Petal Dance: Roserade's sidearm. If you want to throw out a Grass attack for the next three turns consecutively, this is your attack.
The Other Three: Not Roserade's strongest move, but it's decent if you just want to lob a Grass attack out there.
Roserade has the option to Growth, which will now provide a +2 boost. Roserade doesn't employ it nearly as well, though, due to lack of Chlorophyll.
Physical Attacking @ Life Orb
Seed Bomb/Razor Leaf
Natural Gift/Sleep Powder/Body Slam/Return/Double-Edge
With Technician, Roserade has a greater diversity in its Physical attacking area. Compared to Breloom, Roserade has approximately half the power, but significantly more Speed, and also a different STAB option...
Notable Move Illegalities:
Bullet Seed + Technician
Body Slam/Double-Edge + Technician
Spikes + Sleep Powder
Pre-Evo Corner: While I wouldn't use Budew in LC, I have actually used Eviolite Roselia in OU with great success.
This Roselia takes hits extraordinary well, especially Special hits and especially Water hits; not even strong Fire attacks can faze Roselia since I also combine it with a Politoed. These moves are just four out of a plethora of defensive options.
Cotton Spore is a new move, but I don't think you'd find any real use for it.
Roserade does not have anything outstanding in Double matches outside of Water Sport or Worry Seed; it doesn't even have any resistances to back itself up. I suppose you have Technician boost on Round?