Our focus is mostly on making life easier for the DM, but I'm curious what sort of features you felt were missing and which game system you played.
It was a 5E game, but it predated your 5E wotc license. (So I'm withholding gripes about 5E specific stuff.) Also it's the middle of the workday and I'm posting from memory, so this ain't exhaustive.
Why couldn't I ping the map?
Why couldn't I add/subtract numbers from existing values on my character sheet? (this isn't the same as editing a value)
Why couldn't I log in and edit my character sheet between sessions? (obviously, I'm aware of the limitations of the client/server architecture--but that shouldn't prevent me from having a local copy of the char sheet to fiddle with, especially given how obtuse the spell/powers scripting is.)
Why couldn't I generate random numbers that don't conform to dice? (e.g. 1d73)
Why weren't there tooltips on the buttons to let me know what to expect? (more of an interface problem than a missing feature, but eh.)
Perhaps the answer to some of those questions is "You can, but you don't know how." In those cases, just replace "this feature is missing" with "this feature is obfuscated behind an unfriendly interface to such an extent that no one in my group knew how to do it."
Looking back over this post, I find I don't like the irritable tone I'm adopting--I hope you don't internalize it--but the sentiment is sincere, as this eight month campaign left me frustrated and dissatisfied with your product.