Funny, I've seen this thread before, but I don't think I've ever posted in it.
I wonder if Toady/Threetoe ever read it anymore, though...
Anyway, I can say this much about the game:
The greatest problem for a new player is the "Sandbox paralysis" problem - there are a hundred things to do, a maze of menus, but no clear idea of what is important.
A clear indication of what is important to the player (if nothing else, a "?" button based menu that says "what order of things should I do first?" would be a good idea) would be the first step. That is, dig a hole or build a wall to keep everyone safe, and start building a farm and some basic housing to start. Just cover enough to keep them from starving to death or dying to a badger infestation or the first goblins, and then let them figure the game out from there.
In terms of interface, by and large, the overwhelming problem that this game has is in two fronts:
First, this game's interface is designed from the perspective of what is easiest to code, not what is easiest for the player to grasp. Building and designations should be more "close" to one another in interface. Interface should be organized based upon function, rather than what they involve actually doing - putting workshops off of a given build key, and then a traffic management sub-menu instead of doors and floodgates and bridges all separate, and having a furniture direct placement sub-menu, for example. Players should get an idea that there is a menu for major terrain changes like mining and constructions, and then a totally separate menu for traffic flow or designations that relate to forbidding or ordering a tree chopped down. Putting "chop this tree down" in the same menu with "dig a stairwell here" in the same menu with "throw away this trash" but putting "build a new stairwell here" in a totally different menu is just confusing to new players.
Second, and far more important, is that information screens (which have no cursor memory, and take as much as minutes to scroll through sometimes) are often not on the same page as the decision screens, or even close to them. This is most unforgivable on the military screens - where I often have to take pen and paper notes on what dwarves I want to assign to what squads with what equipment, because the menu is SO utterly obtuse.
It should be an Interface mandate that ALL information that could be important in making a decision either be available to the player on that screen, or else be only a single button-press away, and if they press a button to go into a sub-menu or screen giving more detailed information, then the game should remember where they are on that list when they pull back out of the detail view. For a game where so much detail exists and can often be so important, there is no excuse for not helping the player to make informed decisions.
You also need to get over your allergies to charts/spreadsheets in general. The game needs more densely packed information, and using color-coded icons, like a blue/cyan/green/yellow/brown/red happy face icon next to names to indicate happiness problems from a units menu in order to give more compact information that is critical to fortress management in a single menu where players can check fortress status at a glance for major problems would do wonders, because if a player has to go digging through individual details pages of dwarves in menus with no cursor memory, let me tell you, players just won't do that - they'll fall off a cliff.
Losing is Fun only if you learn from it, and feel like it could have been avoided, and now they know what to do next time. Tantrum spirals that keep happening and people don't know why aren't fun. I don't have tantrum spirals, I keep my dwarves ecstatic at all times. But then, I would naturally make super-elaborate fortresses with all the amenities, anyway, as it's just my playstyle to pamper my dwarves for its own sake, and more importantly, I have Dwarf Therapist open at all times showing me at a glance what my status is.