One of the main issues that gets in the way of being able to use a Joomla 1.0 module in Joomla 1.5 using legacy mode is the way that Joomla 1.5 creates the module folder structure.

In Joomla 1.0 when referencing a script the the script reference looked something like this:

That would reference the script nicely since in Joomla 1.0 all of the module files are in the module directory as per the image below.

In Joomla 1.5 however each module has its own folder and so you need to make a change to the way you reference the script accordingly. The previous script reference will now look like this:

In most cases this will fix issues where the javascript, css or image files arent being picked up by the module in question.