I didn't have the time to read this whole thread, so apologies to any redundant suggestions. Some things I think would help:
Better/more clear info in reports and descriptions. Like instead of a battle report that just reads: "The spinning alder logs strike the woodcutter in the leg crushing it!". How about "The woodcrafter fells the alder tree and the falling tree crushes his leg!" Or in a military report for a creature being attacked by marksdwarves you get a lot of "The flying bolt misses the bat!" before one strikes but there isn't anything indication about who shot the arrows. Maybe an option to read a gamelog style combat report with slightly more specific info along with the individual creature logs? It makes group battles a lot more cohesive when reading group combat as opposed to a goblin's combat log where he could punch 1 dwarf, get kicked by a second, and maim a third while the log can make it look like a one on one fight. Making combat logs more specific (without making them even more bloated with text would be really helpful) I like reading them but it usually becomes a burden when I can't tell who was fighting who, so I just give up. Even a way to make the current combat logs clickable would be good, click the line of text about the swordsdwarf who was injured and see their name and wounds/thoughts/whatever is helpful. This would also help a lot with the werebeast problem, if the game would make it easy to find out who was bitten exactly.
Mentioned early were the corpses, after an Elven invasion you get a lot of "Iwame corpse" type items that won't tell you if it's an elf or a giant fly or anything about the creature. How about added at least the race/species, and preferably that species description and their civilization and job, or really any info we could have read about the creature before it was a corpse or bodypart. It matters if you want to dump elves and butcher their mounts.
Corpse stockpile vs refuse->corpse stockpile. This is so confusing, there are so many ways to make that more intuitive- sentient stockpile, citizen stockpile, intelligent creature stockpile, take your pick. How about an option to only put butcherable items into a stockpile and skip the need to deselect each of the hundreds of 'learning' creatures you can think of from your butchershop's stockpiles, and not fill it with teeth and other junk that is useless.
A way to simplify item names in menus. It's nice to be able to read the items description and learn about it, but it's really not helpful when you have pants called "Tileth Sneakcavern the Stormy Gore's Forgotten Beast Silk Trousers" (from my latest game) because all you see in the menu is "Tileth Sneakcavern the Stormy...", it could be a head, silk, clothes, a weapon, or anything else. When I see one laying around I have to pull up the items description just to find out what it is, it should be the opposite, I should see what it is and bring up it's description to see it's composite materials. I'd rather look at an items description to learn about it than have to scan dozens of variations of "trousers" looking for a different item in a stockpile. It's more helpful to see I have 20 pairs of trousers than 20 different names for trousers.
Make some obvious assumptions for the player. If a noob designates a downstair across multiple z-levels just assume it's first a downstair, then up/downstairs until you get to the last level, then an upstair to end it. Digging stairs frustrates all noobs, myself included for a long time.
Make the DF Hack 'Planning Mode' feature for building furniture a default built-in ability (also change the name 'building/build', it's not helpful to anyone trying to place a chair, nobody 'builds' a chair into a room, you build a chair in a workshop- it's not intuitive and that is a really big deal. You build a wall or a workshop, you place or install a chair or bed.) When a noob wants to put a bed in a room it's maddening to not be able to, but have the option right there, not grayed-out on the menu. Allow people to place furniture without having it yet, but give them a message telling them "You don't have any beds to place here, build one out of wood (select trees to chop with [d -> t]) in a Carpenter's Workshop (built with [b -> w -> c]), then it will be placed here by a dwarf with the furniture hauling labor activated."
Grey out unusable options in menus, and/or make menus filterable or sortable. I'm thinking of the forges, I still don't use them from the workshop menu because it's annoying to choose the material first then the item, only to find out I don't have that material or that item isn't available in that material. Also make searchable menus like the managers screen smarter. If a user is searching the work 'door' show all the door options, even the ones like portals with different names, grey out the ones that the user can't use currently.
Make some sort of furnace/crematory/bonfire/pyre workshop for trash disposal. It's a huge burden dealing with corpses and trash, with no good options. dumping it into lava works but it tough for noobs to figure out and same with atom smashers. I don't have a problem with atom smashers but they are unintuitive, there should be a way to designate things for destruction instead of a work around people have to find in the wiki.
Allowing cleaning orders, to mark an area with a rectangle or whatever for cleaning, either once or an ongoing order. I've had the same mud in parts of my fort for decades, but I found my tavernkeeper cleaning the road through the desert recently. A lot of people want to build perfect forts and cleaning goes a long way towards feeling accopmlished.
Related- a way to designate things for hauling. As in, this item is a hauling priority, get it before dragging more boulders from the mines. Same as with cleaning, it gets annoying seeing clothes everywhere and dwarves hauling useless items instead of something you want to see moved. The answer now is using DF Hack for auto-dump, Noobs don't know that. Also give some information about why an item marked for hauling is sitting for months - "This sock is marked for hauling but there is no available stockpile space for socks."
Slightly less noob-problem. A way to prevent dwarves from decorating XXitemsXX from "decorate goods with ..." designations from the manager or workshop. Actually making these orders more specific would be great, and I'm not sure why it isn't. I'd love to set a decorate specific items with specific items types, like "decorate swords with forgotten beast bone".
Some kind of "you need more shirts/shoes/pants/drinks/food" message to let you know what kind of problems your dwarves are having. Then info on how to make them, like "you need more shirts make an order at your clothier" or "you need more shirts, build a clothier workshop to make them" type thing.
It should probably be harder to accidentally trade wood to elves.
YES, some give the trade screen some indication that an item contains wood, or some way to filter out wood containing items like you can with mandates. Yes it's funny to piss the elves off without knowing it's an issue, but it's not fun to spend 30 mins tediously trading, carefully avoiding wood and still piss them off and get nothing from the effort when you can't even find the item that was problematic in the first place.
Less job cancellation spam. Especially with burrowing, it only makes finding useful information more difficult when 500 jobs are all cancelled and clog the announcements. Can't you make those jobs suspend or pause until they are available again without throwing logs at the player? Also jobs being cancelled and removed because of some issue, I still haven't even been able to figure out why this happens but when a construction job (wall, floor) is straight up cancelled and removed that is VERY unhelpful, it should always suspended instead. When it's cancelled and not suspended the player has to see the announcement and find where the job was supposed to be and recreate it. A way to find suspended jobs would be a big help too. Right now I usually just run DF Hack unsuspend all frequently and hope it works.
A lot of people mentioned difficulty levels/mitigation and gradual introduction of challenge, so one thing that can help with both of these is to somehow draw newbies' attention towards the existing settings in d_init.txt
I disagree, this defeats the purpose of making the game more friendly to noobs by saying "teach the noobs to change the game instead of making the game more playable". Making Easy-Medium-Hard settings that change the init.txt, or adding a GUI that allows noobs to change individual settings IN THE GAME, that will edit that init.txt is a much better option to keep the game friendly to new people. Telling new players to start editing game files in order to play the game more playable is 100% the wrong move, make the game change those setting itself. You could probably even (continue to) hire Meph to make something like his masterwork GUI to change normal game settings saved in the txts without having him dig into the source code.