When using the new Search Engines Optimization features provided with SharePoint 2013 the browser title for the particular page will replace all contents of the PageTitlePlaceHolder Content Place Holder. This is important to know particularly when building hierarchical titles.
It is a good practice to have your website work with both www and non-www domain (ie. vs. ) domain. Only one of those domains however should be defined as canonical domain for your website.
When referring to URLs in your website decide if you want to use URLs with or without trailing slash (ie. vs. and use them consistently throughout the whole site.
When using multiple URLs for site associate the public URL of your website with the Internet Zone.
When building custom Display Templates for Content Search Web Part always build a corresponding XSL Style Sheet and link it to your Display Template.