Google Sitemaps - easily generate XML sitemaps. http://www.arnebrachhold.de/projects/wordpress-plugins/googl...
Lockout - good for when you F* something up and dont want anyone to see your site temporarily. http://wordpress.org/extend/plugins/lock-out/
Robots Meta - lets you easily control what is followed and indexed by search engines. http://yoast.com/wordpress/meta-robots-wordpress-plugin/
Simply Show ID's - shows page ID's. Something WP should do, but doesn't. http://sivel.net/wordpress/simply-show-ids/
Simple Redirect - makes 301's really easy. http://www.scottnelle.com/simple-301-redirects-plugin-for-wo...
Gravity Forms - a super powerful form plugin. It is not free, but well worth it's price. http://gravityforms.com
http://zemalf.com/713/most-essential-wordpress-plugins/
though zemalf also discusses an alternative list:
http://zemalf.com/1407/must-have-wordpress-plugins/
Other ones that I would install:
Contact Form 7 (if you want people to be able to contact you)
Some kind of sharing plugin like addthis, addtoany or sociable
I so wish I had a resource like zemalf when I first started my Wordpress blog 2 years ago. Many posts on the site explain how to do things on Wordpress that you really should be doing, with detailed step-by-step instructions. I especially liked the series on Optimizing using W3 Total Cache and MaxCDN - though you may not want to worry about Optimization until you get to a few hundred page views per day.
Note that you also need to choose a theme. I suggest using a theme framework. CSS and PHP get customized in your child theme, so when you upgrade the framework or Wordpress your theme won't break. I use the thematic framework but there are others (http://codex.wordpress.org/Theme_Frameworks).
That said, you may benefit from things like Disqus or Facebook comments and social sharing plugins, if you don't want to code up the pieces yourself.
It's true that many go on and talk about 20 "must" plugins, when you really must have only a handful. But none... bad advice.
With that said, truly must haves are something like these - Cache (W3 Total Cache), backups (e.g. WP-DBManager) and optimization (WP Smush.it).
And unless theme provides the functionality, not using XML Sitemap generator and some SEO-plugin is throwing away free traffic.
Plus personally, I consider Google Analytics for WordPress a must, as it makes special tracking easy/automatic (e.g. views per category), that would require theme tweaking otherwise - in addition to adding analytics tracking code handily to the blog.
p.s. Using cache has very little to do with traffic - and one of the biggest mistakes people make is to start thinking about caching when they already have traffic - what if something goes wrong when you're getting 500 visitors a day? Wouldn't it have been smarter to do all that when you had 1 (yourself)? - The site just works better with cache and optimization, even if just one person reads it - and the few visitors like it more because it's fast.