See apple.com for your answer. I can't believe on hn you'd ask why anyone should upgrade. Being on the latest version is more secure for one. People can stick with old versions, but then they'll miss out on the new stuff. For some that doesn't matter, but most of hn are early adopters so using the latest would be the norm. While osx has introduced some annoying changes, for those with the skills, or the ability to use google, they are easily disabled.
Looking at apple.com's list of features for mountain lion
Lets see what was added in mountain lion that I would use:
* Improved preference pane (OK, this might be good)
* Accessibility pane available from anywhere (No)
* New braille display support (No)
* Voice Over stuff (no)
* AirPlay Mirroring (Don't have anything that can use that)
* Auto Save (Don't really want this, I use vim for most things anyway)
* Built-in Sharing (Might be cool, but meh)
* Calendar (I use google calendar, so no)
* Contacts (I don't use this)
* Dashboard (I don't use that)
* Dictation (Cool, but no)
* Dictionary (Not really using this either)
* Facebook (Nope, not using any of that)
* Features for China (I don't need these)
* Finder (lets take a closer look)
* Inline progress for downloads and file copies (Ok, not bad, but I don't normally mount any servers or stuff. I can just scp stuff that I need, or git pull)
* Easy encryption (Yay, this is neat)
* Share button (Nope)
* Customizable sidebar (Huh, wasn't it customizable before?)
* Tap for Quick Look (This is neat, I'll use it a few times)
* Game Center (Dont' care)
* Gatekeeper (Don't want)
* iCloud (Not going to use)
* Launchpad Search (Don't use launchpad)
* Mac App Store (Nothing really great in here)
* Mail (I use gmail, so don't care)
* Messages (Don't use)
* Notes (Might be cool, but I don't think I'll use.)
* Notification (I already use Growl, don't really need this, it's just going to be another thing to configure. Maybe I'll like it, I don't know)
* Photo Booth (Now you can add pictures to Twitter! Don't need that)
* Power Nap (Might be cool)
* Preview (I use Skim.app, so don't care)
* QuickTime X (Don't really care)
* Reminders (Don't really care)
* Safari (I'm using Chrome)
* Security (Some stuff might be good, nothing super important)
* System (Nothing too great in this section)
* TextEdit (I use vim)
* Time Machine (I only need one place to backup, so the multiple places to backup isn't that important right now (might be in the future), but the encrypted back ups are nice)
* Twitter (Don't care)
So I get:
* Improved preference pane
* Power Nap
* Easy encryption
* Tap for Quick Look
* Time Machine
Not really worth the price of updating.
I guess I'll update once the next one comes out, since they fix multiple monitors with Mission Control to work more the way I like it.
That might be true for 10.7 to 10.8, but 10.6 to 10.8 is a bigger gap. It's all that you listed (and most users would gain more than that) plus ASLR, TRIM support, resizing windows from any edge, AirDrop, improved Finder, much better multi-touch support, the recovery partition, etc.
I was reluctant to upgrade from 10.6 to 10.7 because I really disliked the changes to Spaces/Expose, but even last year it was becoming more and more common to see apps dropping support for 10.6. With Mavericks around the corner, 10.7 will start seeing the same treatment soon. At $20 to upgrade, it's more than worth it.
