1. Merging of menus. The ability to look at things being spread across multiple keys creates a lot of clutter, when you could have a look menu that then has secondary keys that allow you to look into different parts of what it is you're viewing. Also, the menus absolutely need mouse support. (and left clicking to look at something would to me make sense as a default)
2. As with the above, menus can be renamed and reordered to make more intuitive sense. So, select build, for example ; you would then see workshops, furnaces, furniture, mechanisms (relabel traps), constructions, etc. This would make things less confusing for new players. Move digging into the same menu set, as well as things like smoothing stone under a "stoneworking" submenu. Even if it means more clicks, it being more organized would make it far more accessible to new players.
3. Lack of information. Looking at something should have an interactions screen, that shows you, for example, if you're looking at livestock, whether it can be milked, sheared, butchered, etc. and list what workshop it can be done at.
4. Expanded weaponable / armorable alloys. This is the point that had me make Orichalcum in the first place, but a number of alloys that one would think a dwarf could use for something, can't be. For example, you're able to make silver and copper weapons, but not sterling silver weapons. Brass was historically used in weapons, and is nearly as strong as bronze, being far superior to copper, but dwarves can't use it for arms or armor by default, and sometimes you have a ton of zinc on your map. Similarly nickel, cupronickel was used historically by the chinese in "white bronze" and is comparable in strength to bronze. Platinum being an amazing weapon material but you're restricted to using it by default to decorative purposes rather than practical ones even as a siege is in the process of breaking you. So on and so forth. The metals you can work with should be expanded to make maps less hit or miss by the roll of dice on the map.
5. Difficulty options on base world gen that on the easiest setting toggles off megabeasts, werebeasts, the amount of the world covered in evil biomes, etc. Leaving advanced generation the same, but a "peaceful" lowest difficulty setting that would restrict the generation of any sorts of innate threats to your fortress from afar would go a long way to helping out new players.
6. Give the expedition leader the abilities of a manager without the need for a desk and the like. This would more smoothly let players plan things out to be constructed in the beginning of a fort as needed before infrastructure is in place.
7. Rework of the orders menu into the management screen, to be compiled with other policies in place, such as enforcing mandates so you don't accidentally trade things, or automatically queuing mandated items or not.
8. Moving of cutting down trees and gathering plants on the ground to the zones menu, letting you automate keeping paths clear, maintaining a tree farm (perhaps with a submenu of "cut after tree reaches a certain number of z height units), and preventing dwarves from traveling too far from the fort.
9. Military overhaul. The entire menu system needs to be reworked - especially scheduling.
10. Refuse and corpses should honestly be combined by default under the stockpile system. Also, dwarves should be less sensitive to the corpses of enemy combatants.
11. As others have mentioned, a grazer rework is in order, grazers should be able to wander and find food if they are not in a designated pen / pasture.
12. Stairs and ramps need a tutorial at the very least explaining how they function.
13. Fishing needs to be reworked. Fish should be less likely to be caught as populations dwindle, an announcement posted when it's reaching such, there should be a zone setting to leave a minimum percentage of stock to reproduce, bodies of water that go off-map should be effectively inexhaustible, (at the very least, if it's a major river or an ocean!) and fish cleaning should be given higher priority than fishing soas not to leave seafood to rot when the dwarf in question has both tasks enabled.
14. Cooking all your booze and/or boozable plants by accident is a mistake I made many times when I was a noob. I'd set it to have booze cooking disabled by default, as well as any food items with both options for cooking or brewing defaulting to having cooking disabled.
15. Construction / channeling dwarf AI could be reworked. Have it run a check to see if building any parts of a construction would get in the way of building other parts in it, then setting up a queue. This would let us at least put down lengths of walls or designating an area to be channeled out without making it inaccessible.
16. Pathing optimizations. Greater multithreading, perhaps a seasonal check of ideal routes for designated tasks, pre-planning wildlife movement on extra threads before they come to map, etc. I don't know if any of these are already done, but pathing is by far the thing that seems to lead to fps check, and while pathing priority already exists, perhaps having an automated version of it that adjusts every season by default would help optimize the game more, while still allowing players to manually designate preferred routes. (but also preferably showing players where the automated preferred paths are)
17. Groundcover like grass and floor fungus shouldn't block access to sand/clay. Dwarves should be able to dig up tiles to access such - I mean they can already just by placing a dirt road. Not having to go back to a patch of dirt to build a road over every few seasons would be a good reduction of tedium for experienced players and confusion for new players.
18. constructions should be more intuitive. Constructed floors shouldn't block constructed walls, a straightforward way to work around the present system might be perhaps to have the system queue up a deconstruct order immediately alongside the construction order when one is placed in overlap?
19. Dwarves should not get stuck in trees like cats. If a dwarf is stuck in a tree with no way down, they should be willing to risk a broken leg jumping down in favor of not starving / dehydrating to death.
20. Trees should be knocked down / collapse if a tile is dug out from under them, and should not grow on tiles where there is a tunnel dug beneath them, soas to prevent holes in the ground giving easy access to your fort, and / or at least alerting you to the presence of said hole in the fort.
21. Announcements need a filter.
22. A colony opinions screen that shows things your fortress needs or your dwarves are complaining about would be quite helpful to noobs and those wanting a less tedious experience.
23. The embark screen needs to be reworked, the just embark haphazardly should be renamed to haphazard embark or randomized embark, to help new players realize they should be planning things out.
24. Stairs; Up/down stairs should be able to be designated to dig on the surface. (leaving down stairs) to make it more intuitive to new players.
25. Tattered clothes should either have a stockpile of their own, or go to refuse piles by default.
26. Workshops should be able to be assigned wheelbarrows. This would go a long way to cutting down wait times on stone and the like.
Edit: 27. Also, bugfixing in general needs to be prioritized. There are so many long-standing bugs that need to be addressed, some of which go back a decade. Please, take a good chunk of time to fix a number of them, given many can be game breaking or accelerate FPS death. Others are just unending sources of frustration.