There are plenty of things I miss from old editions. Most have been mentioned already in this thread. But in all honesty, I would not change a thing. 5e serves its purpose well getting new people and interest into the hobby. This edition is for the new generation of gamers who are not burdened by the wants and wishes of editions past. I have plenty of old material and options to fulfill my wishful needs for a more complex and dynamic game, and the experience and wisdom to combine and adjust the rules from every edition to suit my tastes.
More importantly, if 5e gets too good then no one will ever want to try different games. Lately, I have been finding more people wanting to talk about different systems because they have grown bored with D&D. I can relate to them having been there myself. If it wasn't for 5e, I might not have discovered some truly innovative and alternative styles of play that have become my preferences; namely the Star Wars RPG, which has rekindled my passion for Star Wars in general, and more board games.
For this reason, I think 5e is perfect with all the missing pieces. Rather trying to scratch every possible itch, I think it has finally settled on a niche and has agreed to stay in its lane. Let everyone customize to taste, and allow gamers to try something different without trying to satisfy every alternative on their own. That just creates more division within the community, which it still suffers because we can never let go of our past. Let 5e be 5e. And let more gamers discover more games.