What tradeoffs? Dude, the rMB doesn't even surpass the rMBP 13 in battery life considering how the intel core M processor runs on 5watts. The new rMBP 13 has way more computing power, better iGPU, and sports 10 hours web/12 hours mp4 video playback.
The only tradeoff buying the rMB is paying $1299 for one and $1599 for a weak upgrade (512GB SSD). Sure the thing is lightweight and completely all metal but seriously, you gotta be kidding me if you think this rMB is not high margin profits for apple. 2/3 smaller logic board, less aluminum, cheaper screen panel pricing (lower res) and of course let's not forget no ports except a mic and a USB-c and lastly the lowly 480p camera that's gonna break the bank for apple
If you can afford to buy one more power to you but if you think this rMB at this price point is worth it over any rMBP 13 (2015 model), you are illogical. Even though we're talking 1.5lbs heavier at most, the rMBP is already very light. Don't even compare to MBA that thing is on its way out the door soon enough. TN panel in 2015 is a laughing stock.
I will wait and see in person how good this 12" rMB really is. I really think it's just an iPad AIR with a permanent keyboard running OS X.