Two interesting games tonight.
First game, I randomed Puck. I do alright mid versus Pudge (should've done better), our other two lanes start dominating. We're ahead ~20 kills by this point, take their mid rax, everything's looking good. Suddenly, and seemingly with no transition period, their Alchemist goes from "Shadowblade + stun then die" into "HOLY %&*$ I'VE GOT BKB + AC + BASHER" and proceeded to murder our entire team. Our Lion getting caught out of position 3 times in a row didn't help, to be fair. We lost a game that, honestly, we should've won.
Second game, I randomed VS and swapped with a randomed Warlock. Again mid, again do alright (versus a pretty decent Invoker). Their team has a Huskar, though, and after a few mediocre fights we're in danger of losing. Then Jugg starts getting solo picks, our Slark starts murdering, and I manage to farm an Aghs. The rest of the game consisted of me dropping the rock somewhere near the fight, right-clicking our fountain, and microing the golems while Slark + Jugg ran around killing stuff and my hero prayed that he'd run back to base. We won a game that, honestly, we should've lost.
This may provide some justification to Valve's stance of "no, we aren't adding a concede button", because I'm pretty sure both of those games would've ended much earlier, and with a win for the opposite team, if surrender was an option.