I haven't played the old version much, but I can venture some answers I got from trying to step back in time.
1) 7x7 in both dimensions with no support. A hallway can be 6 wide and as long as you want it without worry. A 7x7 room with a natural column in the middle is perfectly fine. Just makes sure you never have a unbroken 7x7 square of open space and you're golden.
2) There's no natural underground farming plots, so you have to irrigate land to build a farm. This is accomplished in two ways. One is waiting for the cave river to flood and leave mud behind - it automatically does this when you breach it, and will flood randomly a couple times a year. The other way is to irrigate a room yourself by
a) Build (yes, build) a channel from the river to the room you want to irrigate.
b) Build a floodgate at the end of the channel, and connect it to a lever.
c) Seal off the room, pull lever, then pull again and wait for the water to dry.
The danger is that putting a floodgate on a channel makes it into a water cannon - it will never run dry, and water moves a lot faster in 2D.
3) Lots of little reasons; easy, accidental, permaflood being one. Vicious wildlife is another, 2D is where the legends about elephants and mandrills come from. Creatures will also periodically attack from the cave river, the magma, and the chasm - snakemen, gnomes (who pull levers), even GCS rarely. There's also the little problem that after you start mining adamantium (if you chose to), a timer starts that guarantees your fortress will end - not danger or threats, just an automatic death clock.