Couple of suggestions from a heavy user of Roll20 ...
Too many menus and all look slightly different: you have left and right panels, the menu and the hover tool bar. Put the menu at the top of the right panel and, if you close it, have a simple There appears to be no way for the DM to fix a mistake such as a player accidentally ending their turn early.
Some data entry tasks (e.g. setting character scores) have unnecessary steps, like I have to click "Set Strength" instead of just having a pull down. And I have to set each stat individually and click update instead of setting all 6 and then have one update button. Stats really should also have a point buy and standard array options, most people I know use one of these.
Same will spell selection ... each spell slot is a selection instead of a "Pick 6 1st Level Spells" or "Pick 2 Cantrips" multi-select.