Looking across both the basic and advanced customizations provides some idea about the extensibility of the platform. A commonly asked question at this point in the architecture discussion is, "Does Alfresco have an API?". Actually, it has several. Let's look at what APIs are available and where they are used. This should also give you some idea as to the tools and skills you'll need to have in your toolbox as you embark on your own projects.
The following table shows the APIs available and where they are used:
As the list of APIs shows, knowing Java will be the key to just about any successful customization effort. FreeMarker and JavaScript are important, but are easily picked up using Alfresco's code and online resources as references.
Alfresco has a vision for a web client with a much richer interface. At one point, the plan was to build the web client entirely with Adobe Flex. Alfresco has since backed off that approach. It is more likely that Flash components will be added where it makes sense.
From a skills standpoint, it is still uncertain how deep Flex skills will need to be to customize Alfresco as it evolves into a richer interface. Hopefully, Alfresco will abstract the configuration and customization of the Flex-based components such that clients can get it without Flex skills. If that doesn't happen, it should be fairly easy for anyone with knowledge of JavaScript and XML to pick up Flex skills.