• It’s fast. You open the program, it displays.  The quicker I can get to my webpages, the quicker I can get to work.
• It maximizes screen real estate.  I hate having my usable browser area cluttered by a bunch of toolbars I don’t want that came bundled with other stuff I did want. (Internet Explorer, I’m talking to you).  Chrome even goes so far as to use the top window bar (which is normally not used for anything except the program’s name and window controls) to put my tabs in when maximized.  I love that!  Every little bit of usable screen real estate helps when developing websites on my small laptop screen and I appreciate having those 20 or so vertical pixels back.
• It comes with great developer tools.  FireFox has an add-on called Web Developer that I truly love for debugging websites, especially CSS.  But the built-in developer tools that come with Chrome blow it out of the water!  You can change CSS properties on any element and watch how the rendering changes, without re-uploading the CSS file.  And you don’t have to download an add-on to do it.  While I was writing this post, I used the real-time CSS modification abilities of Chrome’s developer tools to fix a problem with my WordPress theme that was chopping off the title of my posts if they got too long.
• Did I mention it’s fast?
• It’s simple.  It shows me what I need, and hides what I don’t.  It doesn’t bury clearing the cache in some tab in some dialog box, under the heading Temporary Internet Files (again, IE, I’m talking to you).
• It has all the keyboard shortcuts I loved in FireFox and had gotten used to as a Web Developer.  Things like CTRL+SHIFT+DELETE to clear cache and cookies (essential for web development), CTRL+T for a new tab, CTRL+SHIFT+T to re-open the most recently closed tab (handy when you just closed something and realized you weren’t done with it yet).
• Did I mention it’s fast?

Anyway, that’s all I can think of for now.  The list is in no particular order, and is not terribly thorough, but there you have it – my unofficial list of why Google Chrome beats the pants off of any other browser out there.  I still keep the other browsers installed on my machine to make sure the CSS I’m crafting works across browsers and doesn’t break my nice site layouts, but as far as day-to-day heavy lifting goes, Chrome is my new best friend.