I have not looked at MooTools in a while either. But here are my points for JQuery:
- Consistent programming model (there is a JQuery way that works)
- Excellent documentation. When I started JQuery had the best documentation out there.
- Extensive 3rd party plugings
- Microsoft support -- I am an asp.net developer, this helps ease clients minds. Plus it ships with my tools now.
- Lots of getting started guides.
- JQuery's website looks nicer than MooTool's web site. I'm sorry that matters, but it does. Remember, many of these tools need to appeal to designers as well as developers.