http://www.reddit.com/r/rpg/comments/2l69tp/ama_mike_mearls_codesigner_of_dd_5_head_of_dd_rd/cls8996
I feel like this hits my main complaint as a DM. I don't mind optimization at the table and expect it, however it can suck what makes the games entertaining for me out of the game.
No offense but it sounds like straight-up, "kids these days" grognard elitism. My guess is it's what Mr. Mearls thinks (probably with very good reason) that his target demographic wants to thear. Not that I mean to paint your observations with the same brush, though...
Usually it leads my players to want to speed through the levels because they want that next optimization point over just having fun playing the game - including failing and having things go sideways.
I think you can roleplay and roll play but I may need to make a motivational poster that says, "Be interesting and entertaining" on the wall.
In the 90s there was a Role not Roll debate going the presaged the intensity of the edition war. It's not any more valid now than it was then, and, as you observe, you can absolutely do both. You can roll the dice and still RP. You can optimize, mechanically, to fit a character concept, or even just be to 'efficient' without sabotaging the game for everyone else. You can apply system mastery without spoiling anyone's fun.
It mostly just requires a little consideration for your fellow players.
4e is a favorite system, but Mike's comment matches what I saw at the table - players asked for options and then did the exact same options every single encounter. The script repeated fight after fight. I found it very unentertaining as a DM. PF and 3.5 went very much the same way.
Find a good effective combo and repeat ad nauseum.
Relatively few combos are equally good in all situations, and RPGs, by their nature, are very open to variety. Players and DMs - groups - can get stuck in ruts or hemmed in by 'group think,' but they can also break free of it. The more the system empowers players to make meaningful decisions, the more it needs to keep those decisions balanced to avoid 'obvious best' ruts. The more latitude the game gives you, as the DM, to vary situations without needing to change the system, the easier it should be to experiment and shake them out of those ruts.
In 3.5 you can use a targeted dispel magic to break up a lot of combos, for instance. In 4e, you can't repeat a problematic encounter power every round or a daily every combat - they're not like problematic spells that you can keep casting until you run out of slots or charges in your wand or whatever.
In 5e, you can shake things up just by ruling against the combo the second time (or whenever you notice it getting tired). Fighting system mastery with system modification. That may not be what you're looking for - I think you're more looking to shift the culture of your group - but it's an option in DM-empowering 5e.