But I was wondering, how many of you have attempted to make up your own system? Even just for fun or to see what it was like. Did you ever run it? If so, how long did the games/campaign/whatever last? What was your experience? Did people who played with you actually like it? Did you learn anything interesting from it?
I did.
Before I ever got into contact with rpgs I created 'copies' of card and board games I enjoyed. Initially, my main motivation for this was saving money. It helped me to improve my creative skills (drawing, etc.). More importantly it helped me to strip down a game to its core, recognizing what was just fluff and what were the important mechanisms that made the game work.
Later, I also started tweaking rules and came up with a few original concepts for new board games, some of which I implemented and playtested (with moderate success).
When I first got my hands on one of the Fantasy Fighting books, I wrote one myself (a small one), slightly tweaking the concept: I didn't like that in most situations you couldn't turn around and revisit previous locations.
After my first rpg session (back in 1984, playing DSA) I immediately got to work to recreate what I remembered. It didn't work very well, though. There were too many new concepts that I didn't have a clue about. It was quite wonky and the math was completely off.
After being introduced to AD&D 1E and playing a few sessions things got decidedly better. I started creating an rpg system that more closely resembled how AD&D had been advertised to me:
Mainly I wanted it to have a more flexible magic system and more tactical options in combat. The system that finally evolved from this was quite a hit among my friends and we played it for about three years. It also inspired at least three of my players to create their own rpg systems, though only one of them survived longer than a couple of sessions.
What basically killed my system was bloat:
Whenever I came into contact with something I liked (movies, novels, other rpg systems), I started to integrate some version of it into my system. In the end I started to loathe the crazy hodge-podge it had become.
I was also not satisfied about the simulationist aspects of the game. While I did a lot of research to create a somewhat realistic setting it simply wasn't enough. It was also sometimes difficult to get the required information (a problem of the past in the internet-age).
In my world-building phase I became interested in pretty much everything.
After that I made several attempts to design a new system but they never survived the concept stages. Time (or rather the lack thereof) also started to become a problem:
I became frustrated because I realized it wasn't really possible to create a competitive system single-handedly and balancing required lots and lots of playtesting (and willing playtesters).
So, I ended up collecting and analyzing 'professional' rpg systems available on the market. Which is pretty much what I'm still doing today.
It helps that I enjoy mathematical problems. One of the first things I do when I dissect new systems is calculating probabilities and making statistics. But this interest in math predates my interest in rpgs.