For starters the software doesn't crush your machine as iTunes does to Windows.
Second of all, the physical player doesn't magically get a 'corrupted OS' or 'unhappy folder face' problem once a year - requiring trips to the Genius Bar.
The software does everything iTunes does like recommendations, folder tracking, etc - except you could get 'all you can eat' music for $10 / month.
In general the Zune HD device is just sick. Metro UI, intuitive touchscreen, and most importantly, completely unbreakable.
I have to second the completely unbreakable part. I have dropped it whilst biking many times (seriously I'm embarrassed to say like 4 times) and it has only a few minor dings to show for it despite tumbling on asphalt at 10-15 mph. It seems like the thickness of the aluminum bezel coupled with the thinner screen width protects the screen better than the ipod touch's. I also prefer the UI and basically what it came down to when deciding between the two for me was what I wanted: a really good touchsceen mp3 player or a more generally useful device/portable gaming machine. I already have my phone for the latter purpose so I went with the best mp3 player.
I did have mine "break" but I can still use it. Somehow the screen cracked from the inside. Speculation is the battery was overcharged and swelled. I looked it up on the Internet and pretty much every discussion about it was filled with people claiming those with broken screens must be lying or had damaged it somehow. Microsoft doesn't acknowledge a problem.
Oh well. It still works, and I have my WP7 now. The Zune is just for biking.
Adding to the list of everything everyone has mentioned (which I also agree with), a major factor for me is the Zune Pass (and it the main point where I mention Apple is so terribly far behind). $10/mo for (quite nearly) the entire catalog. They have SmartDJ (I think Apple has the Genius Match or something? Similar idea) where you can pick a song and it will play similar songs to it. With the Zune Pass, it will pick songs from the Zune library as well, possibly songs you've never heard. It's great for finding new artists with a similar feel to the bands you love. Years back, this is how I found the Black Keys. Plugged Seven Nation Army into SmartDJ and there was Stack Shot Billy.
If you cancel, you lose access to the songs after 3 days, but each month you get 10 songs without DRM to download and keep forever. At 99c per song, you're basically paying for one album per month and getting the rest of the library for free. And if you're a little more creative, there are ways of making the songs "lose" their DRM.
Everyone I know that has had a Zune, really loved it. They praised it. The people who didn't have a Zune talked about it negatively on the basis that it was a Microsoft product and no one in their right mind would invest in a Microsoft product. They usually have no future. Sadly, this stereotype rings truer and truer with each passing year.
Prior to the iPod Touch, I liked Zune's navigation and found it much more intuitive and useful than the wheel that iPods used. The software was also good and it played music just as well as the iPod.
I think the UI/interface for the ZuneHD was much better than the iPod Touch as well, and the software is still better than iTunes (although both are kinda behemoths). The iPod Touch wins on having the App Store though, the Zune just really never competed in that area for me. Now I go with an Android phone, and with Google Music, my ZuneHD hasn't been used in forever.
For me, it's the seamless integration of both playing and retrieving music. If I'm listening to the Black Keys, I can press their Artist name, load up a list of all their albums, then choose which one I want to stream or immediately download to my device (with a $9.99/month fee, like Spotify etc.)
Last time I used an iPhone they had a complete separation between the music player and the iTunes Store. Never understood why.
