You should always have a link to the home page and the previous page on every page. True facts: a lot of people browse the web on an iPhone, and the iPhone doesn’t have a back button.
Make stuff behave the way that people think it’s going to behave. Don’t make it look like a button or a slider or an expanding accordion section if it doesn’t actually do anything.
A corollary: everything that does the same kind of thing should look the same. All buttons styled similarly; all links styled similarly; all menu items styled similarly; all icons looking alike and actually doing something.
Instructions shouldn’t be necessary. But even so, you have no idea who the user is. Maybe your ideal user just handed his iPad to his grandma, and she doesn’t know what to do. Make sure the user can access the instructions from EVERYWHERE.
If you’re trying to sell me something, make it easy to buy without going through a separate registration process. Make the “create an account” process integrated with the checkout [so it doesn't feel like I have to go through extra steps before I get to pay for my nail polish or dog sweater or gourmet caramels]. Or let me use my Paypal/Twitter/Facebook to create an account. It’s not hard.
Error messaging matters. If someone messes up during login, don’t just tell them “An error has occurred. Please try again.” Did they mess up their password? Do they need to create an account? Is it something wrong on the back end [and therefore not their fault]? Don’t make them guess.
Test it on mobile, and make fixes where it’s necessary. Smartphones and tablets are not a joke, and they’re not going anywhere. Even if you don’t have a budget for a full mobile site, at least make sure it’s minimally functional.
Anyone who habitually makes their links target=”_blank” deserves to be lashed to death with a million wet pad thai noodles. Don’t fucking do this. If a user wanted to open all links in a new browser tab, she’d set her browser preferences to do that for her, or use the keyboard shortcut. When you make a decision like this for the user, you’re putting your own needs for pageviews and increasing average time on the site above their experience.
Obfuscating the content only gives you a short-term win. Nothing kills an experience like having to wait through an unnecessary interstitial ad before you get to the actual content, or having to close a modal that’s urging you to subscribe or buy an ebook because it’s blocking what you actually came to read. HelloBar is a great alternative to this if you want to promote a product or drive subscriptions, or you can make your own HelloBar-like overlay. Just don’t hide the content, okay?
Don’t justify your text. The web is not a newspaper, no matter how much you wish it were. I’ve been guilty of this in the past, for sure, but it makes the text significantly less readable by creating odd-sized spaces between words [and sometimes even spacing out the letters, if you haven't set a letter-spacing value in your stylesheet].
What kind of UX stuff do you notice when it’s missing?