WordPress has an odd way of handling category templates. You are supposed to create a file called category-wildlife.php for the ‘wildlife’ category. While that is intuitive and simple, suppose you have 50 categories on your site, and 3 category layouts?
Maybe you have a magazine-style WordPress theme with a slider at the top of the front page. The issue of pagination comes up when the user hits the Older Posts link.
This is a simple way to make a site more like a smooth web-app: give a half-second transition property to all links.
If you need a Paypal donation button, and you want the user to be able to select either one-time or recurring donations, its not so simple. These require completely different form submissions to Paypal.
Wordpress comes with 2 taxonomies built in: Categories and Tags, but you can add Custom Taxonomies if you like. I was building WP sites for three years before I needed to set up a custom taxonomy for a client.
If you want to put some related posts in a widget you can use the smart YARPP plugin. Or, if you merely want to list posts with matching tags, you can put a function in your Wordpress theme, or make your own plugin.
After you make the front page of your theme responsive (see my previous post), you need to check the other templates. And one area that might break your theme, and resists the simple CSS-media-queries method, is the comments form, typically loaded on the single post template.