Big mistake a lot of people do is try to go for flanks too often, had a reaper the one day that was trying so hard to get a team wipe with their ultimate that they were hiding on ledges for several minutes, we were playing with a man down and one of the our only picks that could do real damage we lost while he was standing on a back ledge near the end of Kings Row while we were fighting to get to the check point.
I notice that a lot when playing with a friend that just got the game, we get a mix of people around my level and his level, quick play I notice we have 3 people trying to flank sometimes, so each encounter we were always out numbered and then the flankers would dive in and get destroyed in 6 on 1 or 2.
Roadhog is always going to be good in a game about getting picks, widow used to be a great character for getting picks but the game has changed enough she isn't as good. Roadhog is the only character that I feel I have to adjust my character pick and how I play based on his presence or lack of presence, if their roadhog is decent you have to play more conservative in how you peak and poke at them.