I am still writing up my con report. But here are some thought on 5e combat.
I played 5e about two dozen times with four different groups with one of them very large in size (8 to 10 players per session).
My opinion is that there is no single combination of abilities or characters that provides an "I win" in D&D 5e. Sufficient numbers will provide a challenge every time.
I ran running Phandelver for two different groups, one a five man party and the other the 10 man large group. Doubling the Phandelver number wasn't sufficient. It wasn't until I increase the number to four to five times the original value did the encounter have the same outcomes as the five man party.
My takeaway that the teamwork rules 5e. Multiple character working together effectively have a huge multiplier effect. Then again they do in real life as well. That it works for monsters as well as PCs. Even if the characters are carbon copies of each other. The interim encounter guidelines leads me to think that Wizards know this as well. I think they need to play with the number but overall the interrim encounter guidelines have the right idea.
There are some character ability combination that are highly effective against some types of monsters. For example life clerics versus undead.
Preparation and planning are force multipliers exactly in the same way they are in real life. My last two sessions of the Phandelver campaigns illustrate this. In the next to last session, the party had a plan, stuck to it and hit Castle Cragmaw. They aborted the attack after killing a quarter of the goblins and hobgoblins. They didn't suffer much in the way of damage but were very cautious. Also the Cragmaws were largely unprepared scattered about in the module setup.
In the last session, the party did not have as good of a plan. They had one and stuck to it but it wasn't the right one for attacked the now prepared Cragmaws. So they got strung out in the center of the castle being attacked by the remaining Cragmaws.
Despite a little higher in level, and facing a weaker foe (most of the Cragmaws killed in the previous session were hobgoblins) the party had a rougher time.
For the larger group what did them in was a horde of orcs at Wyvern Tor and exhausting all their area of effect spells. The orcs due to their bonus move were able to disperse. Plus they got strung out along the perimeter around the cave entrance. When the orcs managed down one party member it opened a hole and allowed the orc to attempt to defeat the party in detail. In the end out of 10 only two were standing with two permanently dead. This even after a failed morale check took out a quarter of the remaining orcs.
In my opinion 5e rewards tactics in the same way as 4e but without all the detail and fussiness of 4e. That the decreased number of abilities has effectively minimized broken rules combination.
That for groups that don't care about tactics all they have to do is maintain superior numbers in combat to come out ahead. Mainly through by carefully picking the time when to start a fight.