Generally, voices and in-character stuff, but that's also as a player. Keeping good notes, organization, but that's also a life issue, lol.
As a DM, sometimes table management is an issue, pacing.
As a 5e DM, balancing combat. I dunno, it seems to me that combat encounters are either so deadly that I have to reign in or TPK, or complete cakewalks. I'm not sure I'm getting that sweet spot where I am challenging the PCs without overwhelming them. Now, this might be my party as well, my current campaign has a Paladin, Druid, 2 Rogues and 2 Warlocks. The Druid player is the least available, so often the party is in combat without a lot of support. So, the party itself feels glass cannon-y.