Why should I log in?
Logging in lets you save preferences, and gives you access to features that anonymous readers don't have, including a journal, the Friends & Foes system, and comment moderation. Logged-in users' comments start at score 1 rather than 0, too. (Bonus: you get to choose the coolest name you can think of! Well, the coolest name yet unregistered.)
Can I change my nickname?
Sorry, but No. It's just too prone to abuse.
How do I change my password or email address?
How can I delete my account?
You can't. But don't sweat it; the unused accounts don't hurt anything.
I forgot / can't get my password!
I'm having trouble logging in.
What are login sessions?
A login session is a glorified cookie; to provide some added security, you can determine the duration and location for your session. The duration options should be self-explanatory: "Lasts Forever," or "Closes With Browser." The former is good if you're the only person using your computer. If you're slightly paranoid, or often log in from remote terminals, choose "Closes With Browser" instead.
The second decision is your location. The first choice is "Never Moves," appropriate it you have a static IP and you never log out of Slashdot or share your computer. "Moves within Subnet" is useful if you have a desktop and never log out, but your ISP sometimes changes your IP. "Follows me Everywhere" is the right choice for machines that move from network to network. This option is the least secure, but oh-so-convenient, and it's also the default.
What are slashboxes?
Slashboxes are configurable mini-feeds that appear on the right-hand side of the page. If you're logged in, they're configurable: you can select which Slashboxes (from dozens of choices) show up on your page—everything from single-source news feeds to webcomics to recent tags, etc.