When creating a Joomla site, it's incredibly easy to go overboard with extensions. The incredible selection of extensions and templates makes it tempting to implement too many and to spread your knowledge too wide.
As a Joomla integrator, I find it's crucial to find a method of implementing sites which makes it easy to set up, maintain and document a large number of sites. Moderation is a virtue, a least when building Joomla sites.
For instance, the use of too many CCKs, text editors or template frameworks will lead to constant headaches. Particularly in the documentation of a site, it's important to reuse information. If you're using too many extensions and different processes for each site, the task of documenting and training will be so much harder.