Despite having done a lot of mountain biking in the past, I raced 3 road seasons before I did a mountain bike race. I had an okay bike - definitely not top grade but usable - and had been spending some time on the trails, although not as much as I would like. It turned out that I instantly loved it.
Was sort of "meh" on the cross thing for a long time, probably mostly because I'm a wicked contrarian and when people, en masse, won't shut up about how awesome something is I usually wind up thinking "oh go back to Jonestown you freaks". Sort of had to do it this season. Turns out its awesome. It was not the instant love that mtb racing was, probably because I was SO raw at it when I started. Both put me in over my head at first, but cross is really different from other bike racing you might do. I had to understand it a little bit, and then I fell hard for it.
Before some road races, I get excited. Before a lot of them, I honestly don't feel much. That's not a bad thing - I know it's sort of hip to be a self-loathing road racer. I'm not. It's just that Jeff Cup, Poolesville (despite that I flat there every year), Reston, and some others REALLY hit my switch, and others I do to do with the team or to get races in (although I have sucked at this more every year - my road race count goes down each year or so it seems). Before mountain bike races I just bounce off the walls. I love them. Usually, I wind up going out and trying to press my riding skills two days before the race and I flip over; that's just dumb but it's borne out of excitement. Before cross races, there's lots of I guess you might call it dread.
The scene at a cross race is fantastic. People make some part of a day of it (we made LONG days of it this year) and there's lots of fun. It's also the best spectating, and weird as it is I enjoy pitting a race per day (more than that is tough though). But the races are intense, there's no joking around during them. This year I've been lucky to be doing the 10am race which is the perfect time for me. Get there at 8, set up the tent, get the number, pre-ride after the C race, then get ready to go, race, help the lady get ready for her race, pit for Paul, then have beer and heckle the B and A races. I love the schedule.
Today I rode my mountain bike for the first time in a long time. Cross helped, for sure. I noticed that cross has helped road riding a lot, too. Road helps cross and mountain, there's just nothing that gets your motor in shape like road riding. I think mountain biking helped cross a lot, just in bike handling stuff. Can't really point to a lot of direct ways in which mtb directly helps road, other than the many times when the thought of riding faster on the mtb got me through some workout when my motivation was otherwise flagging.
If I had to choose one type of race to do I think I'd have to go with mtb, with cross making the decision really tough. But if it weren't for road I'd never be in a position to ride well (such as it is - as much as I don't suffer the delusions of others well, I do worse with my own) in the other two. So really, at this point I can't see the whole "bike racing" thing without any of the three.
I didn't forget about track, you just can't do everything.