Flash/Flex

HTML, Javascript, Ajax

General

Flex framework is component-oriented and event-driven which helps in creating rich user interfaces (HTML was not really created to support application UI scenarios)

Javascript is a programming language

Usage

Flash is predominantly used to build a stateful application that runs completely client side

JavaScript is primarily used for cross-browser portability.

Debugging

Flex has a true debugger and profiler within the Flex Builder IDE. So debugging is far easier

Debugger might not be that helpful for JavaScript. As it involves different browser so a browser debugger might be of limited use.

Scripting

ActionScript is a nice mixture of Java and JavaScript so you get a familiar language with strong support for OOPs.

Javascript has different syntax and execution between browsers due to the nature of JavaScript engines in modern browsers

User friendly

Password manager doesn't work,
Text fields don't remember previous entries.
Browser shortcuts like Ctrl+T and middle-clicking doesn't work,
Ttext search doesn't work etc.

No such issue

Client side Reservations

Clients must have flash installed (might need to install, might not be able to)
Clients must download flex framework (though a few hundred kilobytes)

No such restriction exists for JavaScript

SEO

Flex content is not accessible like other content and is also not indexed by search engines (contrary to what Google might claim).
This not only affects people without flash but also search engine spiders. There may be some hacks to help get around this now, but I think that most flash content will never be indexed by Google.

Search engine friendly

Pricing

Flex is costly $$$

It’s free.