Update: This is a really old post; I don't develop apps this way anymore, and I hope no one else does either.
I maintain many web applications that were originally developed strictly for use at what is now a division of a larger conglomerate. All of these ColdFusion applications are running on a single web server, and as our sister companies have become interested in using them I started to consider the implications of an expanded user base. After some analysis I concluded that, though these applications were designed for maintainability over scalability, the current equipment could adequately handle the expected load increase. Once I was confident that the local infrastructure could support an increased load I began to think about the clients.
One of the new facilities that will be using these web applications is located in Mexico, and available bandwidth to that location can get tight at times for a couple reasons. First, the bandwidth on the WAN going into that facility is less than what is available in the US. Second, the traffic is regulated so that the our SAP traffic gets the highest priority, and web traffic gets the lowest. Since bandwidth could be a concern, I started looking for ways to optimize my pages for size.
So, it was from the perspective that MooTools was too sophisticated for my needs that I began to look for other frameworks. After looking at many frameworks (Dojo, GWT, jQuery, Prototype, YUI), I settled on jQuery. Not to say that jQuery is not sophisticated, it is just that jQuery seemed to focus on the aspects that I needed and left everything else to be loaded as a plugin. From a file size perspective, jQuery is only marginally smaller than the MooTools core version that I was using. From what I was reading, jQuery was supposed to be marginally faster than MooTools, but so far I don't have complaints about either.
Besides for technical considerations, there was one other factor that pushed me towards jQuery. The size of their community and the amount of corporate "endorsement" they seem to have. Normally I like to judge subjects on their merits rather than popularity, but the fact of the matter is that if I were ever to leave my company they would need someone to maintain the code. The larger the community behind a framework, the more likely they will be to find someone capable of managing it. Or, if nothing else, the next person who is responsible could find documentation online. I must say that I do personally prefer MooTools' online documentation format, but there just seemed to be more support in general for jQuery.
I am just getting started with jQuery, so it is possible that I could end up coming back to MooTools again. And, I am not ripping out all of the dependent code that we currently have, so I will be working with both and as I move forward I will document my progress.