Here's are some common pitfalls you'll probably want to avoid!

1. Main page content block. This is a block ( structure > blocks ) and should always remain in the CONTENT region of the site, and should never, ever, ever be edited ... never.
2. Promote to front page. Just don't do it. Really, it isn't pretty. Actually, on occasion it looks ok, but please, please use sparingly and please, please look at it before assuming it looks good. Typically the images end up pixelated. A custom block is a good alternative to this.
3. Blog posts. There are 2 blogs available to you. Blog Entry is the Public Site blog (also called news) and should not be used for members related posts. Members Only blog on the other hand is intended for your members only site use. 
4. Menu. Menus are designed to live in their own space ... moving the menu just causes alarming display issues! 
5. Menu items. Best practices state that you should never have more than 5 to 7 menu items. Also, you may notice that if you have too many menu items your menu no longer looks good.
6. Images on responsive themes. If you are adding images into the body of a page or block and you are using a responsive theme (resizes for mobile) then you should absolutely be using % ... check out this handy video