Compatibility, deployment (MSI package), and customization (can configure the server with addons, that in turn help you configure the client much faster than other solutions I've tried). AUTOMATIC NAME UPDATING. This was HUGE for me! This was the main reason I moved away from GoToAssist. I worked with GoToAssist support multiple times, but they couldn't help me get anything in place to do this. Essentially, if a PC name changed, the GoToAssist Server kept it's name listed as the "old name" - this lead to tremendous amounts of confusion for me and made it almost impossible to know which computers needed updating and which were already the correct name, simply connecting to support a user was an issue after just a few computer moves. Deploying software to 50-some machines might not be as large a task as other companies out there with 500+ machines, but as a one man shop, keeping those machines updated to ensure versions match, and names of the PC's are kept intact when users retire and new folks come on board is a HUGE time saver and stress relief after swapping to ConnectWise Control.
Poor screen scaling (when compared to a competitor like TeamViewer), slow multi-monitor (becomes a sluggish because it seems to want to stream all screens at once, even if only one is selected), and ambiguous information as to whether another third party who also uses the software has left theirs installed / potentially could cause exploitation and already causes confusion.
If I connect to a machine with no other users on it, I believe I should have the option (via the "join w/options" link, perhaps) to change the connection to match my resolution (similar to how Microsoft's own Remote Desktop or TeamViewer do)
Additionally, even if it's unable to match my connecting resolution, If the built-in scaling were better (I.E. - if there was a feature that introduced some good hardware accelerated anti-aliasing), the daily use of this software would be greatly improved.
I've seen this with some other remote connection software - even the more legacy VNC protocol suffers from it, but it is avoidable because not all have this issue. I think the software needs to only stream updates to the primary selected monitor - or at least have an option to enable that, because delays become a huge problem when navigating things like complex databases or even simple excel sheets when there's a full-second delay. This does not happen on single monitor connections right now, it becomes noticable on dual-monitor, and on triple monitor it's very obvious.