Give us a match challenge rating
Currently players have no easy way to gauge the difficulty of matches, either before or after the fact. You can try to look up players' ranks, but the MMR used to make the match might be very different from what the ranks suggest. It makes for a lot of confusion.
I understand Riot has reasons for not displaying MMR, but that doesn't mean they can't display the difference for an individual matchup.
Here's what I suggest. At the champ select screen, the client calculates the difference in team MMR and displays a match challenge rating from 1 to 5. A 3 would mean match maker believes the teams are evenly balanced. A one rating means this should be an easy match for you, and a five rating means you have a real challenge ahead of you.
Every matchup can't be perfectly balanced. Adding this data to the champ select screen might reduce some of the complaints players have when they find themselves in an uneven match, because at least they'll be forewarned. And think of how pleased you'll be when you win a challenge-5 match!