I'm kind of perplexed by the drastic changes. I thought about things though and they could make sense.. implying other changes be applied. Anyhow, here's my take from maybe a somewhat different light:
I have played Riven since just before her major hp5 nerf a while back last year. The nerf was necessary and she has still been viable with some tweaks to playstyle. Right now with the new items, things are just kind of crazy for many champs. In my opinon, what it boils down to for Riven is that she is an all-around different playstyle when compared to that of the other most common top lane bruisers. She is not really tanky-dps in lane at all and if you play her as such, you're going to get destroyed. What makes her viable is the quick harass and mobility. Valor in, slap, stun, slap, hop out, blah, blah, until you can finish off the opponent with an ult (if level 6) or a combination of skills and summoner spells. It is all about setting up to catch your opponent making a mistake and that takes some finesse over just raw tanky power.
Also, playing top with Riven, in my experience, is usually feast or famine. Yes Riven has awesome snowball potential, but this is what should really set her apart for her playstyle - not being so much a tanky-dps type. If the idea is to force a "balls to the wall" playstyle by taking away more hp5, we need some other sort of sustain. Yes, lifesteal is a big part of Riven.. but it's a problem when I'm too low on health after exchanging blows to get in and hit minions consistently. Sure I can use skills to get gold from minions.. but I still am not getting the life back that I need to stay in lane. You see, the changes can be justifiable if you are trying to more clearly define Riven's playstyle. But what she needs now then is BALANCE. There are some champs that it is just impossible to counter harass due to their innate tanky-ness, stat synergy with key items, or high ranged damage potential (ie- singed, jayce). Maybe some extra damage to broken wings to encourage harass, a hp5 bonus added to valor when damage is taken could work, or a small buff to the stun time of ki burst to assist getting away, lower cd on valor.. just SOMETHING to make sure I can play a high risk/high reward, high mobility style without going back to the fountain after every little skirmish. This is affected too by ult time increase. If I can't finish an opponent as soon as I'd like, this gives them time to take advantage of their various sustain methods (a heal, inbuilt lifesteal, higher hp5, etc). How am I suppose to go through a phase of harassing again with even less health this time around? It's not possible.
The ult was needed to make sure there wasn't just endless harassment with no finish. With higher ult time and no sustain, I am now forced to play more passively for a longer period of time between cd before I can try to work on harass again. This will essentially lead to me being pushed around profusely and giving up tower damage. Staying in lane is now going to be impossible with no other form of sustain. Overall, you see, without making some other modifications, these changes make Riven very inconsistent. I will close by saying that late game Riven is almost 100% dependent on the lane phase. If I get shutdown in lane, it is no easy task trying to come back and do my job late game.
tl;dr - Early game balance in playstyle is the big deal here.
Also I apologize for grammar/spelling or nonsensical comments. It's pretty late here.