I've been playing DF for maybe three weeks, now, so.... still a relative newbie. I've gone through at least a dozen forts, which I abandoned generally after the third wave of immigrants (like that one time 32 immigrants arrived in a single wave, to join my existing 9 ). The fort I'm currently playing is currently in summer of it's third year, the longest any have survived so far (due largely in part to the fact that approximately 25 migrants met the bottom of my Atom Smasher.)
Like others have said, there is a short massive learning curve to get the basic hang of the game (generally getting used to the interfaced and being able to learn to do the vital things like getting food, basic defense, keeping dwarves happy, etc), after that (assuming your population is small enough to avoid sieges and megabeasts) you can often survive for a very long time. At that point its on you to do one of two things to make it more interesting; you can start to explore the various other game mechanics (make some armor/weapons, explore the various workshops, etc), or impose challenges on yourself (have a higher population count, start in a more dangerous area or an area with fewer resources, limit the food you can get from farming, etc). Fortunately the game enables you to do both of these things!
There are tons of examples of ways to challenge yourself, some I have done recently are attempt to live entirely within the caverns with only basic supplies to start with, embarked on a land mass with a triple aquifer, started on a haunted forest and tried to live above ground (only digging for materials, no living down there), but the limit is your imagination (and lots can be found here or on the wiki).
My question is: How large/populated does your fortress usually get? And, at what milestones?
Feel free to answer that question and ignore the rant which follows it, if you so desire.
I generally set my population max at 110 or so (so I can get 'all the features', but limit the number to help for framerate) but oftentimes I abandon, or am killed, when I am around seventy or eighty. As for how large my fort gets.. In ideal situations my fort has a small above ground area (walled in) for fishing, butchery, and sometimes an above ground barracks, some soil/sand layers for item storage, a stone layer that comprises most of my basic rooms, a stone layer for bedrooms, and generally a very low stone layer for my forges upon striking magma.. So I guess on average, counting the surface, that's like five or so (if you don't count in between layers serving as irrigation or stairways and such).
My 36 dwarfs have a 6-man military which is largely ineffective due to their lack of desire to equip their weapons, although I'm hoping some adamantine armor (which is in the works) will even their chances... assuming they equip it. However, I've not yet had one bad thing happen to my fortress. I realize that not every fort will be a Boatmurdered, but the worst things to happen were a one-time raid by some monkeys, who were put down by a single, newly-drafted wrestler, and a gremlin that my "proficient" miner killed before I even managed to notice it.
Well breaching the caverns is a good way (depending on your embark) to find some 'fun', my last fort had full four floodgate emergency doors which I figured would help some, and it did at first, a bunch of monsters were just hanging out on the other side of them when I opened the caverns, then some blind cave ogres showed up and tore right through them followed by two trolls, and a whole mess of krundles, we eventually beat them back with minimal casualties* but there was some fun (also for fun after a battle read the description). Another way would be to increase your population size which will result in more attacks by enemies, or declare war on the elves or humans.
But yes, if you embark on an ideal site and have a low population count, basic defense, food/booze supply and keep your dwarves reasonably happy you probably won't see much action.
*Amazingly one of my dwarves (I noticed because I named him after my roommate) had both arms torn off (one at the shoulder, one at the elbow) and survived, he mostly just hung around the meeting room what with having lost the ability to grasp things but hey, there ya go. To increase the awesomeness he later fought a giant rat by biting it.. I mean honestly its like they are trying to get me to quote Monty Python. It was the most severe injury I've currently seen a dwarf suffer and still live to tell about it (not killed in the fight or succumbing to infection following the fight). Another dwarf, in the same battle, got the award for most injuries I've ever seen a dwarf take without dying (though he did die of infection following the battle), his block of red text was almost as long as all the rest of the descriptive text put together (amazingly he didn't lose any body parts at all).
This is because I haven't traded anything, right? I haven't needed anything from the caravans except a few bags, but otherwise I've been self-sufficient. The other cause is my low-population, but I find that killing the immigrants is better than the alternative. If I let them live, they sit around drinking all my booze because I can't find anything for them to do.
Explore the caverns, declare war on your traders, build new and bigger projects, play with the military (bugged right now though.. But you can feel your way through it), or abandon and embark on a harder area. Just a few ideas.
As my population increases, I find the only thing I'm doing to take the larger numbers into account is either making a military which comprises 75% of my population, or making them all miners/carpenters to make more housing. I've probably only actually built 1 out of every 10 things in the game, because there has never been any problem requiring them. An excellent example is the butcher's shop- I've never had so many dwarfs (even when I didn't kill all the migrants) that I couldn't feed them with a farm or two. My population of 36 is living off a single farm, and we've got 500 spare plump helmets in our stockpile.
Well perfect chance then, start a new fort (or continue this one) and don't use farms, try just using the butcher shop, or only hunting for food.
Sorry if I sound whiny, but I've been taking in all the information I can off the forum/wiki, and I get the impression I'm "playing the game wrong", by which I mean I'm not experiencing all it has to offer.
You aren't really playing it wrong, think of it as playing the game on 'easy mode'. Every now and again somebody suggests that Dwarf Fortress have difficulty settings, but really it already does, you choose how difficult or easy it is based on what self-imposed challenges you add for yourself. Playing the vanilla game in ideal settings (provided you have the basic controls/concepts down) is very easy. If I set my population to max at thirty dwarves and embarked on a perfect location I could survive a good long time, maybe forever, but as you say it would get boring fast.
A few general tips:
1) Work your population count up slowly to get used to them, start a game with a max of twenty, if you feel you've got the hang of using all of them increase it to thirty, then forty, etc.
2) I admit it is irritating when you get huge stacks of migrants all at once (it does seem like its worse in this version then the older ones, but maybe I was just lucky back then), one thing I do to try to organize them is I have a custom class of Ranger, they are plant gatherers, fishermen, trappers, and haulers. I generally set all my new guys (if they come in a huge mass) to that class, or plebs (peasants), so they are doing something productive until I can get a handle on where I need them.
3) As I and others have said, pick a challenge, either self-impose regulations on yourself or pick a harder area. Start maybe with a haunted forest, or if you want something harder maybe a desert or glacier (though admittedly caverns have in some ways made those two easier as far as the 'I need water!' thing goes). Sometimes haunted biomes come with all creatures being undead, this not only makes them all hostile but also means you cannot use them for resources (yes this includes fish as well as the animals in the caverns). Just some ideas.
-MB