I'm with the "If you don't like it, don't use it" crowd. I have a couple of things to add though.
Magma: As mentioned above, not such an issue in the 3D version since it's rare. Fun, but likely to be much less effective once invaders are flying or more intelligent. Add ladder/bridge building to invaders and it won't be more than a speed bump. I'd also love to see gremlins along the lines of kobold thieves who sneak around and disconnect mechanisms Exciting times, when you pull the lever and find out that the floodgate holding your magma back isn't connected right now.
Traps: Restrict yourself to single-weapon traps or maybe bow traps which must be reloaded to make them less useful. Simulate mine fields by leaving a clear path through the traps and designating the pathing cost of the trap squares as very high so your dwarves avoid them.
Turtling: pulling up the bridges and hiding when there's a siege might seem cheap, but it's currently a personal choice and it's easy to come up with role playing reasons to not cut your fortress off from the world. Fortresses aren't as self sufficient any more either; I understand tree farming requires randomly striking an underground river in the 3D version and while you can cordon off part of the upper world for your wood needs, that's still a very deliberate and time consuming effort to make, which you can just NOT DO if it seems wrong to you.
In my fortresses I always try to maximise opportunities for trade, even if I don't need anything - which requires at least a partially open fortress. It's a personal challenge, as others have said. In the future I would love to see some extra effects from trading, such as happy thoughts from fine imported goods and happy/unhappy thoughts from mail delivery or lack thereof. But I still think it's great to have the choice to cower in fear if that's your style.
I'm sure once the army arc is in it will be impractical to turtle behind a lava moat and send your armies to conquer the world - I expect diplomacy and combined forces will be integral to making a dent in opposing civs, and if the diplomats get slaughtered by the goblins camped outside your fort that's going to be very hard.Toady seems to have a good instinct for gameplay, and the current invasion forces are very much a mildly amusing place holder.
I'm going to post another comment about difficulty and balance between the player and the computer because I suspect most people have stopped reading this one by now