I think this is going to be my fav change of the season hands down.
Being able to play the role I want to play is 95% of the reason I play team builder now. 5% is being able to play whatever champion I want.
So, lets think downsides. The new champ select that is replacing team builder will have bans. Team builder does not. So, that removes that 5% bonus I mentioned for me anyway of being able to play anyone. They did say they will make the blind pick queue just not have bans and use the new system, so that's still an option I'd "kind" of have - kind of meaning that I still can't guarantee I get to play whatever champ I want. Not as big a deal, but it means that any "practice" of new champs, if they have a high ban rate, will likely be restricted to custom games. That's kind of a step back in my mind, but its probably worth the trade offs.
If you are queuing up solo, it also means you are not guaranteed to the role you want.
Anyway, the I think those are the only 2 downsides as a replacement for TB specifically - you are not guaranteed to get the champ you want to play and you are not guaranteed to get the role you want - while you can today. I doubt that's a big deal to most folks, but its something I liked anyway.
That's all pretty minor, but worth noting as I'm trying to think through the pros and cons and why I play the modes that I do.
Downsides outside of team builder - well, bans might end up being straight stupid. 3 people per team each get 1 ban. What happens when one of those people decides GP must be banned every game. What happens when another guy dislikes X instead of more needed ban, etc. I think the concept of 1 ban for 3 people per team is more fair, but I think it will result in some folks that are commonly banned now getting through. We'll see on that one.