Thank you for the replies, and curing my ignorance. So one DM I play with is guilty of a lot of the no-nos you guys listed. (that game has been very frustrating, but lets not dwell on it) With a different group I am running a mind flayer of thoon game, so I've made knowledge check DC's rather high so as to keep some mystery as to what they are facing (I do give them some info, but it is usually incomplete. Also they rolled really well and made a good logical argument so I let them rescue a partially burned out book detailing some things about Thoon) I've also reacted to some of their choices, and give more information then I planned if they ask good questions/are smart. In this regard so far I feel like based on your definitions I am doing rather well in keeping their player agency high.

My worry comes in that they are facing mind flayers....mind control is sorta part of their schtick. Not to mention the party are one bad decision/a bad roll or two from death due to brain eating. This seems like dangerous ground in terms of player agency. Next session is tomorrow, they are only 5th level currently, and they are going to head into the city where the rest of the campaign is probably going to happen. So I am afraid of them stumbling into an area where they'll just die because of getting way over their heads. That is unlikely because I have the flayer lair pretty well hidden/not in obvious location. Also I'm banking on them not being totally stupid.

I started this thread to help me as DM learn what possible pitfalls to avoid and help me grow as a DM.