My biggest complaint with 5e is that it abandoned a lot of the things that 4th got right (and I came from 3.X originally).
The absolute biggest of which is the 'martial characters are incredibly weak compared to spellcasters at any level past 3' problem which plagued all other versions of D&D. Allow me to quote from somewhere, and note that this is the druid, not the wizard, which has the most game-ending potential. http://forum.rpg.net/showthread.php?622716-3e-what-was-the-exact-moment-you-realized-Caster-Supremacy-Share-your-tales-of-woe/page7
To head off the response: ALL OF THIS IS FIXABLE WITH A GOOD DM AND A GOOD GROUP. But it's a shame they just doubled down on a fundamentally broken component.