I kind of agree with the OP in a way; the API for iCloud is already in Lion, so what Mountain Lion so far seems to add is better integration for Apple's apps, not really an OS level change. That may well not be entirely the case, but iCloud doesn't seem a strong enough incentive to replace the OS version when Lion's iCloud integration doesn't really seem any worse.
Notes and Reminders are definitely not OS-level features and I agree with the OP completely on this one; they're just apps, pushing them as a Mountain Lion feature does more to highlight how badly implemented they are at the moment. There's no reason why Lion users couldn't benefit from those apps.
Likewise, Messages is just a weak update to iChat. While iMessage support is nice, it's still a pretty poor IM client compared to much easier to use apps like Adium.
Share Sheets are a bit of a weak one too; as it's something a third party developer could just as easily create as a library for app-makers to include (or have their apps look for). If anything I'd rather have less social networking integration in my apps, but maybe that's just me.
Gate Keeper however I disagree with; I think it's a much more important feature than it seems at first glance. For one thing it's just deliciously simple, and yet in that simplicity suddenly your average user is no longer as likely to install apps from malicious sources or, more crucially, apps pretending to be genuine. While there's still a question as to how quickly Apple will revoke a malicious developer's signature, it's a far more elegant solution than built-in anti-virus (which requires a lot more work to maintain) or dynamic sandboxing etc.
Airplay Mirroring is a nice feature, though with Lion/Mountain Lion's still fundamentally broken multi-monitor support the shine kind of wears off pretty quick, plus it's not going to be of use to a huge number of people.
I do also agree with those waiting with baited breath for WWDC, as I've always been more interested in an OSes behind-the-scenes features, which we haven't heard a great deal about yet, so I'm still hopeful that Mountain Lion will have something of real interest to me. Currently the only feature I'm looking forward to is GUI support for multiple time machine targets, but when a simple script can do that on Lion I'm not too sure about upgrading just for that
1. If you ask some developers about iCloud they'll tell you that there's not enough documentation and that some features don't work as easily as they expected. Proof positive about this is Apple's own apps which don't use documents in the cloud for the OS X versions. That should be a hint for us all that Lion's iCloud is subpar.
2. Notes in Lion still uses IMAP for syncing and doesn't support photos. Reminders is poorly crammed into iCal and uses CalDav for sync. Apple never claimed these features were revolutionary but they highlight the easy sync capabilities via iCloud. The statement "There's no reason why Lion couldn't benefit from those apps" is confusion on your part. Lion does have Reminders and Notes but they're shoehorned into awkward places and are skimpy on features. Apple cannot change mid stream in Lion and suddenly change the UI and how these apps sync.
3. Why would I want a 3rd party to do Share Sheets? What would be the value in that? Share Sheets are everywhere. In Quick Look, in the Browser and available to all OS X developers. There's no 3rd party vendor that give give developers that scope.
4. Air Play mirroring - You're wrong on the usage. Lot's of people are going to use Air Play mirroring. If I'm a presenter all I need is a TV/Projector, A wireless network an Apple TV to stream my presentation right to the screen. Me personally I wouldn't have to worry about HDMI adapters for my MBA (which works flakey with audio) to stream video content to my TV that doesn't support Air Play a la Amazon Prime streaming. Air Play on the Mac is going to be a big deal and will sell a lot of Apple TV.
I'm upgrading for
iCloud for finally cloud enabling my documents
Notification Center
Better Video support via AV Foundation
Better behaving apps via ARC
Game Center
And the bunches of little things that will make computing better.
There are really no glaring features that are missing (except for maybe iSCSI support for networking geeks). A new file system would be nice but not yet a pressing need like it will be in a few years. I think multi monitor support will not likely improve much but what we'll see is larger displays with extremely high resolution (4k)