My biggest complaint is that I usually have to grind group events to level up in the late teens. The XP rewards need a little tweaking it feels like. And group events happen pretty frequently, which takes the uniqueness of stumbling into one away a little. But they're major XP sources.
As you mentioned already, I usually retrace my steps if I find myself underleveled for something. Chances are that I missed some events and stuff. I see a lot of people just take off running as soon as an event finishes but many events are actually chained on previous events so sometimes it pays to hang around for a minute while grabbing some ore or whatever. Exploration, gathering, crafting and doing the personal story all make a big difference. I hit 100% in a zone and it gave me a massive chunk of exp as well.
Just a funny aside but I kept wondering where all of the karma vendors were in beta. No one explained that the renown vendors turn into karma vendors when you've finished their events. I don't know how I missed that, whoops! I have like 2300 karma stockpiled already.
Playing: XCOM 2, Diablo 3 Watching: American Crime, Regression