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.
No offense taken.
Unusual Dice. You can make custom dice strings now, such as 1d73. Type /die 1d73 and you can save it to a hotkey slot if you want by dragging it to the hotkey bar. This may have been added after you tried it.
Tooltips. This somewhat depends on the ruleset but many of them have tooltips. Again, I'm not sure when you tested and whether or not they were added after.
Ping Map. Currently active token from the combat tracker initiative is highlighted to draw focus to it. Players and GMs often use pointers to highlight a specific area or to demonstrate line of fire and whether or not there is cover or blocking terrain from one corner of a square to another area... per the PHB rules. It also measures distance this way, which is nice. I guess it would depend on why you want to ping the map. There may be a different way of doing that in FG.
Temporary Edits to abilities. There is a simple way and a more advanced way.
Simple Method:
use the /mod command and save it to a hotkey. type /mod +2 (Bull's Strength bonus) and drag it to a hotkey. Click the hotkey or press the F1 or whatever each time you want to apply it. It will show the rolls and any modifiers in the modifier stack on the final calculation.
Advanced Method.
Create an effect that is linked to a spell or ability that you toggle on and set the duration in turns, minutes, hours, days, etc and then toggle it on from your character sheet or drag it to a target it if is a spell that effects a target.
An example effect might be:
STR: +4;
The result would look like this in the chat window when you had that effect active and made your next attack:
[ATTACK (M)] Battleaxe [EFFECTS +2]
It would not apply the bonus to a shortsword attack since it is a finesse weapon or to a longbow because it is ranged.
You can make even more advanced effects that combine several features, such as:
Rage;ADVCHK:strength;ADVSAV:strength;DMG:2, melee;RESIST:bludgeoning,slashing,piercing;[SELF]