For me the biggest Issue at the moment is the ATC. The common complaints are phraseology and strange behavior but that is not the main issue and can be worked around somewhat.
The reason an ATC exists is that it guides traffic. This seems to work somewhat for ai planes, since i had to go around multiple times and had other planes behind me do the same. This also seem to work for Taxiing. How well is another story, but this is nothing that is a huge issue.
The huge problem is that there is no interaction with other players at all. It is okay that I cannot collide with other players but It is so important to hear their interaction with the atc and have it recognize other players. The same goes for callouts on airfields without any atc coverage. I want an actual reason to do those. If there is no change anyone will ever hear those, this totally kills my immersion and makes it a chore.
Vatsim is no alternative to this in the slightest since the coverage is way too low (not to blame them, but its a fact) and I don’t want to be bound to the few airports that have coverage.
Additionally the coverage is even certain daytimes.
For many other aspects of the sim 3rd party developers can offer solutions but in this case that won’t happen, since everyone would need the same add on which is not possible.
Griefing can happen, but should be not a huge deal since the only thing needed is the same communication you are already doing with the atc. Simply offer an option to mute players that are spamming or limit the amount of messages being able to sent in a certain timeframe and this wont be an issue.
Until then this is no real Multiplayer.