Thanks to Hirazi Blue for suggesting this flashback.
The initial goal of project MoonDust was to develop a new particle simulation system for XSI. The existing particle system, which had been in XSI since v1.0, had reached its limits. It was a monolithic system that wasn’t flexible or customizable enough (eg available events and attributes hard coded into the system).
I never had to learn the old particle system. There weren’t that many support cases about it, and MoonDust was always just around the corner. I just remember the old particles as a jumbled forest of PPGs and parameters. See for yourself…here’s some screenshots of the old particle system:
It’s interesting to compare how things worked before:
and after:
However, the scope of the project eventually grew into the development of a procedural graph system.
eX-SI 
This is a great comparison. The old system was very inflexible and hard for even a seasoned artist to navigate. ICE is great, and I always loved the project name.
The one thing one mustn’t forget is that the old/legacy particle system was never intended as one of XSI’s main selling points, as far as I’m aware. It always has been a top-notch software mainly for doing character animation. ICE has grandly opened the floodgates to new ways of doing other stuff, but before that most people seem to have used the software mainly for its high-quality animation tools… And were quite happy with that…
I started using the old particle system when XSI 6.5 came around. And in hindsight it’s easy to see, that it was terribly clunky. It cannot really be compared to ICE, but it did its job in its day. And that was, at the time, the only thing that mattered 😉