Combat should be tactical, unique and challenging. Allowing you to have the kind of fights you don’t usually get in campaigns often.
There should be good progression. We shouldn’t stay at the same level for months and higher challenge should equal higher reward.
It should not require specific schedule or be time intensive. You should be able to jump in and play once a month with people who have been playing multiple times a week and your PC won’t be totally useless.
There should be a little bit of friendly competition. Whose PC kicks the most ass? 🙂 Which team of PCs have completed the hardest things (bragging rights).
It should be a social community, ideally with ways to impact or aid each other rather than feel like a siloed or solo experience.
Broken things are not fun. We should try to avoid and/or remove things that make it unfun for majority or that work against the other core objectives.