1. GS is more like free skiing and the initial learning curve is not as steep, because (among other things) the gates are not as close together and you don't have to deal with flushes. (As in, oops, just flushed my chances for getting a good time on that gate combination...) There are some additional wrinkles to slalom (cross blocking, potentially fairly extreme fore-and-aft weight shifts, shorter skis that make fore-and-aft balance and recoveries more challenging) that also make it harder. And all those freakin' turns. I mean, really, why mess up a really fun, nice potential Super-G run by jamming that many poles into it? And slalom (like racquetball) has a tendency to magnify skill differences in timing results of skiers who would finish closer together in other disciplines. I find slalom less forgiving of some of my mistakes. And for those who haven't raced before, there can be a certain overwhelming quality to coming over a dropoff and seeing an absolute forest of slalom poles below. (By contrast, GS seems like a piece of cake: Left turn. Then right turn. Got it.)
2. GS turns are the foundation of high level racing in all the other disciplines--in downhill and Super-G (with the addition of a good tuck and the ability to deal with rollers and air time) and (more than in the old school days) now in slalom.
3. Still, it depends hugely on your preferences, what speaks to you, what your personal style is and how you like to ski. I like to go fast, I like longer radius turns, and a day on GS skis after a couple of days on slalom boards is like getting let out of a cage. (The only part of slalom that really speaks to me is getting to whack into things, and that's not even as enjoyable as properly brushing the GS gates.) You, on the other hand, may live for short swing turns and love the trees and bumps, so maybe slalom speaks to you. If so, someday you should tell me what it says...
(Yeah, I know, "Dean--stay early, get your weight forward at turn initiation, be outside ski dominant, and have fast feet..." But somehow that soundtrack never plays for the whole course. Must be an equipment problem, the helmet blocking out the sound of all that good advice.)