I can't think of anything off the top of my head but I would have noticed certain nuances that would have been hard to hear after being lost in the master mix.
But for me this didn't start with stems alone. It started when I listened to extended mixes and heard sounds I didn't notice before. At first I would dispute it being original and consider it an extra overdub since it was louder or I just never noticed the sound before. But then I would go back and listen to the radio mix and suddenly it was there and I had become aware. Become aware, lol. :-)
For example, INXS "New Sensation" has some synthy melody on the end, that stands out in the remix. John Waite "Missing You" features this wooden "knock knock" sound early on in the extended mix. Never heard that before but in the radio mix it is in there around the second half of each verse.
I realise I didn't give an stem examples so hopefully not too off topic. ;-)