Rory, I think, once wrote something about how the art you start with will determine a lot about how you think (if it wasn't Rory, I don't remember who it was).
That thought returned to me recently for some reason.
The first three things I studied with any reasonable seriousness were boxing, Aikido, and Fencing (specifically the SCA variety). Of those, Aikido and Fencing were the more serious, or at least, the more in-depth in my initial exposure (my initial boxing training was mostly confined to bag work, pad work, and conditioning, with no sparring). I'm at a point where I no longer practice any of those arts, but the lessons I learned from them really informed how I think about martial arts and fighting.
Lesson One: Fighting is Athletic
Boxing and Fencing both taught me that when you start fighting, it's very much a physical activity. Technique and all that matter, but great technique and a shitty set of lungs makes a harder fight. I know that may seem like no great revelation, but if you listen to some people pontificate about fight theory, you'd think that martial arts are about academic debate. Sometimes, you just have to let things fly. Be prepared.
Lesson Two: Hit. Don't Get Hit.
Fencing teaches you something very important--you cannot trade shots when you are fighting with three-foot pieces of steel. If you get hit, there is a very good chance that you will die (in the particular formula I was training under, a single hit could "kill" you). Aikido draws upon a tradition with a similar focus on the dangers of live weaponry, although it mostly brings it into the empty handed realm.
Boxing, of course, does not deal with weapons, but the importance of not getting hit is still taught (at least, in some places). I once overheard a coach of mine say to someone (I'm paraphrasing for brevity) "If you don't mind getting hit, you'd make a lousy boxer. I hate getting hit...I try really hard to avoid it."
This lesson is one of the reasons I'm so fanatical about the importance of footwork. If you can control where you are, you can control how hard it is for other people to hit you.
Lesson Three: Less Is More
I learned five punches from my boxing coach (six if you want to count both uppercuts): jab, cross/straight, hook, uppercut, and an overhand right (which I think he only taught me because I'm relatively short).
Fencing taught me four parries, a lunge, and a couple of cuts. There were some other tricks I picked up along the way, and I eventually learned how to fight with two rapiers, but the core was pretty small.
Aikido's empty handed syllabus is ridiculously compact (I never got far into the weaponry, but even there, it's not super extensive). A reasonably athletic person could probably learn the whole thing in a day (not be GOOD at it, mind you, but learn it).
I learned very early on that pursuing newer, shinier techniques wasn't the key to victory. The key to victory was getting better at the basics than the other person. Again, if you wonder why I hammer the basics, this is why. The basics work. That's why they're the basics. The way I learned, if you can't make the basics work, it's not because you need a new technique, it's because you don't know the basics well enough.
Lesson Four: Make It Work For You
No two boxers fight the same. Nor do the same two fencers. A buddy of mine and I who trained together, mostly under the same teacher, developed radically different games. We had different psychologies, and different physical attributes. Even in Aikido, I learned that there were some things that worked great for me, and some that were really tough for me to apply. I still had to learn it all, but I knew which things I liked, and which things I didn't.
Those lessons have pretty much informed the rest of my martial arts career and training. I can think of worse ones to build a foundation on.