For the love of God, KEEP the GOOD STUFF from 4e!!!!!!!!
I have been playing a ton of Pathfinder lately, and I can tell you what 5e DOES and DOES NOT NEED:
1 - No Save or Lose spells. I don't want to go to my friend's house to roll a d20 and then spend the rest of the night watching them play D&D because I'm dead, stone, paralyzed, whatever for X rounds (which can translate to hours or more of real time). Keep the 4e save every round mechanic.
2 - Keep the 4e action economy. Playing PF and being able to do essentially one meaningful thing every round since so many actions are Standard, sucks. 4e does this much better with its Standard-Move-Minor setup. Everyone can do multiple meaningful things every round. They can attack but still do things like move, open doors, drink potions, heal, sustain powers, etc.
3 - SELF-CONTAINED MONSTER STAT BLOCKS!!!! - I should never have to look up a spell, power, ability, or item in another book to run a monster. NEVER. 4e got this one right.
4 - No fiddly BS rules for things like grapple, or crit confirmation rolls. Pathfinder did a good job if streamlining things with the whole CMB rules. 4e does an even better job with its simple conditions and its grab mechanic. Its easy, simple, and works well in play.
Bottom line, 4e does about 85% right, build on the foundation of what works. Don't throw out the baby with the bathwater and don't go back to a 3.x baseline.
Where it lost me is that classes feel too rigid and structured, and magic items are too fiddly and don't feel magical. I also don't like that 4e movement powers force you to use minis. I like minis and 4e is the best miniature tactical wargame I have EVER played. But the system needs the flexibility to handle both. Also, I think the roles should be more like templates that can be overlayed onto the classes.
Oh and I like options. Don't oversimplify the game. I like having a fighter with multiple powers to choose from. If Essentials had been 4e when it originally came out, I would have stopped playing D&D altogether. There is a need for Essentials, but core 4e for me is still the game as presented in the PHB1.