I am putting together a tutorial that I plan to update with those Liferay customizations that are commonly needed and I have the opportunity to test.
Liferay is a big monster and people get afraid of it when they first meet. It takes time to get used to the way customizations are done. You have the extension environment for some core Portal changes like changing the landing page, you have theme development to change the creative of the site, layout development to arrange portlets, the Control Panel Settings option from where the company icon can be changed and so much more like applying different loook and feels even per page.
As my goal is a quick introduction and not to write a book I will go little by little documenting the most common challenges a developer first face when trying to Customize the monster.