The main thing I need to switch to 5e, both as a player and as a DM, is a superior play experience.
This means, as a DM:
1. Easy to prep. This means both having good prepublished adventures, and good tools to prepare stuff, like toolboxes for environments, monsters and NPCs.
2. Easy to run. I don't want to have to track a lot of stuff, monster, trap and NPC statblocks should be self-contained (no cross-referencing) and short, and I shouldn't have to constantly flip through rulebooks.
3. I should be having fun DMing. Ideally, playing a PC or DMing should offer equal enjoyment. I can have fun playing 3.5, but 4e gives me fun on both sides of the screen.
4. More variety in play experience. I'll confess, 4e, in my experience, really is mainly a combat game, and my group treats it as such. Skill challenges haven't really taken off, and roleplaying in general isn't done much. Now, I know, as a DM or a player, I could be doing more to mitigate that. But if 5e helps me out in this regard, (while keeping the fun of 4e combat) this will be a major reason for me to convert.