I haven't played too much, but one thing that really frustrates me is not being able to browse through all the dwarves easily. Squads have the capability though, so I don't see why it isn't available for every dwarf.
It would be helpful for instances where you want to see if any dwarves are in a particular area or if you want to turn some jobs on or off on more than a couple dwarves. It's very tedious to use the unit list when you are working with lots of dwarves, which seems to occur frequently once my fortresses get over 50 dwarves.
Also, I have another suggestion, though not really a feature I find myself wishing for often, but still useful. A mining designation for veins that would automatically spread to nearby blocks of the same stone/ore type as they were uncovered. It would help with some of the micromanagement necessary to mine veins in areas that you aren't clearing out.
I thought of another thing. When forbidding or reclaiming areas, it would be nice if mining, detailing/engraving/fortifying, tree chopping, and plant gathering designations in the area were also forbidden or reclaimed (assuming they aren't already). At least the option for it would be nice. It would remove some of the steps from trying to convince your dwarves that nothing good is outside and that staying inside the fortress is not only good for their health, but also the only choice they have. Perhaps just the ability to designate wartime unsafe and safe areas, which would automatically forbid all items and designations in the areas whenever a siege happens or when dwarves are forbidden from going outside. Overall, it's an unnecessary hassle to tell your dwarves that they can't go outside and also removing all possible jobs from outside to ensure that they don't run in and out of the gates of the fortress.