A weird title, I agree. Now prepare for a wall of text!
DF is awesome. The game mechanics are awesome, the capacity for creative play is awesome, and the focus on procedurally-generated content and emergent behavior is awesome.
Given that awesomeness, here's my main criticism: Once you learn to build a sustainable fort in a reasonably dangerous environment, the sense of challenge, exploration and wonder tends to depart from the main game and retreats to the margins--defined mostly by staged combats, megaconstructions, self-imposed challenges, and mods. These are all cool enough, but they are certainly peripheral to some extent, no? Why do veteran players need to work that marginalia to have fun?
The reason, in my view: the main tasks of the game which place enormous demands on the new player too easily become rote reflex for the veteran. It's fucking magical the first time you irrigate an underground farm, or conceive of some creatively air-tight defense against invasion, or are finally able to keep your fortress active, fed and happy/drunk. Once you've scaled DF's cliff-face of a learning curve, however, these formerly exciting struggles are rushed through with a few ho-hum and formulaic key-presses.
The new player of DF is always asking "can I?" Can I get a farm up? Can I get a wall and gate in place before ambushers come? Can I keep my dwarves from going mad and bashing each other to pieces? Can my military take on this dragon? Can I survive? The sense of exploration and challenge those questions evoke can absolutely devour one's time once the interface is mastered--it's incredibly satisfying and addicting. Unexpected events, by turns frightening and hilarious, come in a flood to the new player. Creative solutions to problems are wonderfully encouraged--if it seems like it should work, it probably will! In general it seems something fresh is always around the corner.
For the vet player, the exciting question "can I?" frequently degrades to "should I?" Should I negate invaders via a wall of traps? Should I queue up "(P)ull the lever" for this troublesome fetishist of a noble? Should I build a 30 z-level execution tower? Should I civilize this haunted tundra? This happens because for the veteran there's never much doubt of success in dealing with the core challenges of the game--the former flood of unexpected situations drops to an easy drizzle, dotted by frenzied searches for weirdness or novelty anywhere it might be generated by the game (combat, dangerous sites, megaconstructions) or the player (modding, forced challenges).
Part of the reason so many "Should I?" players stick with the game is that the game rewards experimentation and creativity so well--but it's a shame that a certain degree of skill in manipulating the mechanics damns you to search for fun in peripheral things, while the main object (to survive/thrive) is never in doubt. Why isn't it? Probably because the formula for survival in one site/world is virtually identical to the formula for survival in another.
I've seen some argue that the solution is 2D-style scripted events and challenges, but that's ultimately an empty solution--any scripted challenge will simply be absorbed into a player's "winning" formula of play. Each new "scripted" challenge would be fun until surmounted once, and then it would fall to tedium, doubtless generating demand for yet more scripted challenges, and so it would go. Moreover, many of the 2D challenges were due to lack of control or unfinished mechanics--+pig tail sock+ chasing and vicious elephants, for example.
The right solution in my opinion is to use all the variety and interaction already in place to make each site/world play in a meaningfully different way. If I embark on an evil cliff during the murderous and war-gnawed Age of the Demon in one world, I don't want it to be essentially the same as starting in a benign wilderness during the Age of Snuggly Peace in another. My population of dwarves, their personalities and the effects their experiences have on them, should be warped and altered by their personal history, the world's history and their fort's environment/style.
It shouldn't come at the cost of player control, or denying features--it should just add situational challenge/flavor, which the player should meet with the full array of complexity and creative options DF already possesses. A fort of flabby peaceniks shouldn't fight or make weapon traps like dwarves born into fear and constant war, but you should still be able to order them into the breach! (Doubtless to watch them get slaughtered, panic, drop their weapons, lock themselves in their bedrooms, etc.) A noble holding court in a fort sited on a haunted tundra shouldn't enact the same demands/mandates as his luxurious counterpart in a fort sited on a benign mountain valley--he'd probably want a fucking coat, not a black bronze cabinet.
These situational changes shouldn't just be petty annoyances with easily avoidable consequences--they should make it hard for a mature fort to -survive-. The new player has challenges aplenty getting his first 80+ dwarf fort operational, but once there, it would be nice if he were asking "can I survive this?" a lot more
These are worthless ramblings, and it's easy to say "it should be like this" while blithely ignoring the difficulty of the work Toady's doing. He's done a more amazing job already than I could hope for--I'd just like to see the core game demand more and more of a players' skill with the mechanics after he learns the ropes, not less and less.