I think the differences are just what they told us. OIS, larger battery, and bigger/better screen.
The antenna is the same and the body is designed similar to the 6 so there's not really room for a larger/better speaker and the antenna is the same.
I'm glad they do this though, having small differences. Rather than the S5 vs S5 mini (or G3 vs. G3 S, One vs. One mini, etc.) where the hardware is a major downgrade. My fear was the 6 would have a better processor, better camera, more exclusive features, but only the display and battery are different and the only exclusive feature is OIS, which makes the 6 still in the league as the 6+. I hope they keep this strategy and I hope that other companies follow suit (Motorola did it with the mini/ultra/max)