SharePoint has always hidden the real error messages and all debug informations relating the same error, to maintain a good relationship with the final user that most often gets scared at the sight of all those written preferring the usual phrase "an error has occourred".
In WSS 3.0/MOSS 2007 to enable that informations we have to modify only the web.config related to your web application.
Instead, in SharePoint 2010, we have to modify two different web.config:
the one that you can find into the root directory of the web application in which you want to debug you customizations (the file at the path: [drive]:\inetpub\wwwroot\wss\virtualdirectories\[port])
the one that you can find into the "LAYOUTS" directory under the SharePoint root (the file at the path: [drive]:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS)
What remains unchanged from the previous version of SharePoint are the number of attribute to change in the web.config file:
The "debug" attribute of the "compilation" element has to be set to "true"
The "callstack" attribute of the "safemode" element has to be set to "true"
The "mode" attribute of the "customerrors" element has to be set to "off"
