Linux has really, really good package management. You'll realize this the second you try to use another operating system. Ubuntu (thanks to Debian) makes life so easy. It's easy to install all the software I want, and it's easy to make sure it stays updated. Even the kernel, etc. stay updated with practically no effort on my part.
In contrast, managing packages on OpenSolaris is painful. There are multiple package systems, and none of them seems to do everything I want. (In Linux, there are multiple distros, each with their own fully functioning package system.) Reading Joyent's wiki, it seems to be commonplace to fall back to patching and compiling the source by hand. Painful.
Theoretically, Nexenta might fix this situation one of these days, but in the meantime, I have a deadline ;)
Since POSIX is a commodity, the OSs work hard to differentiate themselves. OpenSolaris sets itself apart by being very scalable. Furthermore, DTrace and zfs are supposed to be crazy awesome. Perhaps that's true, but man, I wish I could just get Python 2.5 installed! What's worse is that once I do have it installed, I have no idea how to keep the system updated. Ubuntu sets itself apart by being easy to install and administrate by novice sysadmins like myself, and boy do I feel the difference!
By the way, as I mentioned, I'm using Joyent. I'm not feeling the love today. So much of the documentation on their wiki is stale and misleading. I spent all day trying to figure out how to use Blastwave because that's what the wiki said they used. I finally gave up and submitted a trouble ticket. Support replied (in a timely manner) that Joyent was no longer using Blastwave, but had switched to pkgsrc. Unfortunately, I can't find the pkgsrc directory on the system either. After reading the docs for several hours, I've realized two things: My chances of getting Python 2.5 working and talking to MySQL without immense pain are slim to none, and don't trust Joyent's wiki.