Gotta say, after using Overwatch's client it's real tough justifying Riot's new one.
Being able to use chat during loading.
Having a custom control scheme for every undividual character. (Believe it or not, that's something I'd use CONSTANTLY)
Being able to report DURING and after a game(as well as post lobby), as opposed to forgetting about it 25 minutes later when you have your one and only chance to file the report.
There were some slight adjustments to make some factors of the new client better, but the fact that Overwatch's client is so much better when it's been virtually unchanged since release is a little ridiculous.
Edit: Was wrong on one part I removed about the recently played list. Completely forgot about it, the other 3 points are bigger in my opinion.