What's the downside of using too much JavaScript?

I would like to know what are some downsides using too much JavaScript code in a web page?

For example, I will use a jQuery framework for my dropdown menus, tabs and accordion. And other JavaScripts for my calendar (even-though there is available calendar that uses jQuery) and other JavaScript for other stuff? What is the effect? (My opinion is conflicting of JavaScript and heavy to load)

I know a lot of developers that masters more than 2 JavaScript frameworks. My question is: When developing a project how do you pick a JavaScript framework for that project? Why will you use MooTools or jQuery for that project? (I know that is a matter of choice), but is there other reason? Like mootools is good for this... or jQuery is good for that..

I want to hear you opinion.

What are some downside using to many JavaScripts in a website?

• HTTP overhead
• Low maintainability

Why will you use MooTools or jQuery for that project?

There are specific frameworks out there for specific things--Take Ext JS for example, which tries to encompass everything a site needs. This is not what jQuery does.