I went like 25/1 with Riven before...
Someone on the enemy team said "Riven's not a carry dumbass."
And then I 1v5'd them and won the game.
Handy notes:
All 5 of Riven's abilities scale from AD. This means she has 7 things scaling from AD. Passive, Q, W, E, R, R's second cast, auto attack.
Her passive can lifesteal.
Using her ult makes everything else she has stronger by about 20%, including her ult.
What does this mean? Riven gets tankier and gets more damage output just by building flat AD. Cooldowns and lifesteal.
Her passive is 50% of your AD, this means your attacks are doing 50% more damage as long as you're casting abilities. You don't have mana so why not?
Her ult increases AD by 20%. This means her passive is also being increased by 10%. Technically, you're gaining more than 20% AD.
The only sad part, is that none of this scales on her total AD, only her bonus. So don't get too excited.