The Open Laszlo frame work is an RIA (Rich Internet Application) which is open-source.  It relies on the Flash plug-in in the browser for virtual machine functionality.

Q & A:

What is the benefit of using Open Laszlo as a client versus HTML?

Open Laszlo is a rich client.  This can best be compared to a Visual Basic application or any other Windows based application where the client can maintain state.  Also you have much greater control over application layout and appearance without worrying about the inconsistencies of web standards between different browsers.  As long as the client web browser has a Flash plug-in installed, the Laszlo application can run and will appear consistently across browsers.  No more CSS Layout hell.

Is the reliance on Flash a problem? 

Actually the Flash reliance can be a benefit.  Flash market penetration is somewhere above 97% for the 7.x version of the Flash plug-in.

What about text size?

Actually the application can be split among files so the main application can load quickly and other pieces of the application can be loaded on demand.

Is download size a problem?

Actually the application can be split among files so the main application can load quickly and other pieces of the applicaton can be loaded on demand.

How can you interact with other frameworks and technology?

Laszlo uses XML as the basic unit of data transfer.  As long as you can generate XML with your server application Laszlo can use it.  Data can be returned to the server in any form necessary including the typical form POST method or in an XML format or whatever other method you can conceive.

Do I need any special server software?

No, you need the ability to host SWF files, which should not be a problem for a majority of hosting solutions.  The Flash files are simply files downloaded like any other file (like a picture or PDF) to the client browser.  You can use the Laszlo server which is a Java based stack but I am using Ruby on Rails as my server software.  When Laszlo is deployed in SOLO mode (as my site is) you do not need any supporting server software.  Simply communicate through http to any resource that can return XML.