After 2.5 years into working remotely, the two areas that have been the most challenging for me are 1) cross functional exploration and ideation and 2) user research outside of video interviews.
First, I've found it really challenging to replicate is the ability to get in a room with design, engineering and PMM partners and think through problems and solutions to those problems. Whiteboarding and brainstorming are frequently thought of as startup cliches, but there's something really powerful about getting smart folks in a room to deeply grapple with a challenging problem.
Tools like Figma and Miro can help (I've been so impressed with Figjam and how important it's become to my workflow!) but it's still so challenging to get folks to commit, distraction-free, to online collaborative project time. Slack, emails, pets, kids — it's all a distraction. This challenge most frequently arises during the ideation phase of "how might we address this [agreed upon] job to be done?"
Secondly, and much more tactically for my role at Quizlet in particular, is limitations on observing and interviewing users in the field. My customer is one who works in a particularly challenging environment in COVID world — teachers! Prior to COVID, we used to go into the classroom and see how teachers used Quizlet with their students. I'd make a point to do this activity outside of the Bay Area and visited classrooms in other parts of the U.S., and even internationally. This was an incredibly useful user research activity and I'd regularly learn new things about the problems teachers faced, and where Quizlet could be useful. Due to COVID, we haven't been able to spend time in the classroom and so user research has become much more clinical, with video interviews one-on-one (i.e. no visibility into the student side).
While Quizlet is certainly used directly by students, my focus area has been teachers. For a product where the "buyer" (teachers) may not be the end user (students), solving for only the buyer needs without understanding more about how the buyer and end user interact has been challenging.
Abstracting that out more generally, I'd imagine that for many product managers, not being able to interview and observe users in the field is a big impediment.