1. Read the readme file first, sometimes a theme like Nature needs some adjusting BEFORE using it
2. double check if all relevant theme files are uploaded, usually it works fine through a ftp browser, however some hosts are not that flexible
3. Disable WordPress plugins like sticky posts, themes like Clear Cut with multiple loops won't function with a responsive post at the frontpage
3. posting images with a 500px width in posts, this common mistake will break almost all themes, unless it is a gallery theme built for huge image postings
4. Using long post titles: it's not SEO compatible and did you ever see a newspaper header that long: "Ten best ways how my beloved grandmother cooks sweet cherry beans with salted chips and hot salsa in her old style kitchen."? Well...
5. Using responsive width third party widgets 350pixels wide: it will break almost every sidebar, try to fix the widget within its source code
6. Avoid cluttering your sidebar: it's nice to have your bookmarks, last fm account, used software, twitter, skype online, flickr acount, digged stories, the butt ugly my bloglog code, alexa, technorati, spam counts and visitor stats info exposed, but is it really useful? Placing some of these nifty widgets in the footer area will clean up your filled sidebar.
7. Cluttering your newly established site with tons of ads, Google and Yahoo will punish you, and are ads really more important than your content? You won't plaster your car or home with ads, why you do so with your online presence?
8. Use smaller images in your sidebar, even a fluid width layout will be screwed up, if using 300pixel width images. Firefox can handle this issue, but the beloved IE won't do it.
9. Using dead long menu/page titles: a menu should help visitors, not describing your content, e.g. "Home ⌉ This is the best way to get Windows IE running ⌉ About myself and my cats ⌉ Contact me ⌉ Windows tips to solve screwing"
10. Placing tons of third party widgets, WP Cache can handle your WP Installation, but won't be able to fix third party time outs.
It takes usually ten minutes to solve ten issues, bugs or mistakes, go ahead.