I'm always impressed mroe by a good story and a high level of care taken by an author. Run time around an hour, not that easy to always play for 2 hours straight and I always feel a 2 hour mission would be better off made into a 2-parter.
Not fussed about combat, needs to be a little I feel but I don't mind it being sparse.
IMHO no content 'segment' should go above 30 mins. Create more segments, but attempt to keep the segment itself to 30 mins(optimally the 20-25 range). Tends to give enough time for a multitude of interaaction as well as not feeling 'drawn out' for the sake of length.
That being said, more repeatable content is what I care about. The FE series is great, but I play them once on my 6 characters and that's it; I'm done. Where as something like a new PvP gamemode/map; a new pve 'game mode'; etc etc would keep all my toons interested for 10x the length.
I'd like to see a few more cerebral missions. For example. A two-player mission in which one player must guide the other through a maze like structure. The ceilings of which are made of transperant material and allow the other person to walk overhead and guide their team member by chat or sight or over comms.
I think if foundry missions had more teamplay elements like that then they could attract another niche audience as opposed to simply scaled up/more mobs when you bring a friend.
On a personal note, I usually only offer that fifth star to missions which go to lengths to supplement the story with the environment. For example, going down a market street to get from A to B.... and going down a market street where NPCs use simple speech bubbles to try and sell my junk I don't want. That for me is a small attention to atmospheric detail which supplements the context of the story without any fancy maze gimmicks ^^
I also appreciate if an author takes the time to inject some BO to BO discussion and slaps a few attributes on the mission info, such as estimate play time, content etc.
I have played a few missions which tried to mix things up by offering a break in the main story by going to space from ground. This was justified to me by transferring to the perspective of my Number 1 who had command of the ship in orbit while I'm was off dealing with whatever on the ground. Suddenly, I was playing another character within my ship. It created a change in pace which wasn't forgotten. And this opened up a double storyline, like we often saw in the series. Something where both space and ground work towards achieving some objective or overcoming difficulties the other side doesn't strictly need to know about.