Normal mmr and ranked mmr are separate. I'm assuming you were normals because you had a level 29 thresh...
I had a game last night, my team 29 28 24 24 22 (I was the level 29 as adc and the 28 was support).
other team: 30 30 30 30 26
It must have been because the mmr of me and the support were the highest, but basically we won bot Lane 7-1 (the lone death coming in a 3 for 1 when their jungler ganked), but our 2 solo lanes fed so hard they had us at inhibitor turrets in all 3 lanes by 20 min (we took t1 bot Lane turret but once they got t2 top and mid they just all went bot).
Point I'm making is:
Pre level 30s should not be matched with level 30s in normals, period. If you duo queue then the pre30 gets considered as 30. I've been getting matched with 30s since about level 15, at that point your runes and masteries are so much worse you don't stand a chance even if you're better (not too mention you don't know before the game that they're 30 and have this advantage).
level 30s in normals should be matched by the higher of their normal mmr or ranked mmr (so you can neither have low ranked mmr to stomp normals if matching by ranked, or get way better in ranked then go stomp normals as it is now).
So Yea, I agree, normals matchmaking sucks.