Really it's just how early he can root someone indefinitely. It's not fun to play against, because if you get within 600 range of him when he can proc his passive, you aren't moving for at least 6 seconds. Later in teamfights your team can work around it by CCing him or bursting him down as he can't root everyone, but in 1v1's, it can be abusive. It doesn't guarantee he will win the game, but it does make it extremely annoying and it lacks meaningful counterplay in the early game.
If you're talking about winrates compared to people whining about the champion, look at Janna. Nobody complains about her, but she boasts one of the highest winrates IN THE GAME right now at 54%. And why is that? Because people don't feel like they are getting cheated when playing against Janna (mostly because she is a support, and people just assume it's the enemy ADC that's wrecking their face, not Janna's shield). With Ryze, you can walk into his range and then you are trying to retaliate when he roots you but you just can't while you take his full combo to the face. Imagine if Riot released a champion with a 5 second stun; people would say that is completely broken. But Ryze can almost root you for 6 whole seconds, you have a few fractions of a second to flash away or use an ability to escape between the roots, but it isn't easy for you to fight back against Ryze if you are in that situation.
Now this isn't just QQ. I love Ryze. I want him to do well, but right now he just seems abusive. I feel like Ryze's early game shouldn't be so dominating based on how devastating his late game can be. As it stands, I feel like you need a ranged silence or stun against Ryze in lane to do well against someone who knows what they are doing. Either that or get camped by your jungler for the early phases of the game. I hope Ryze can reach a point where he can be strong but not a dominating force in lane.
Let me know what you think. Good luck out there. Peace.