I recently had the opportunity to see my parents using their computers. They've been "Mac users" since I gave them a dual G4 and a 23" display. They bought their own MacBook after seeing what a refreshingly unpunishing exercise Mac OS compared to the Windows laptop they'd bought themselves against my advice, before I'd given them the dual G4.
Mac OS has multiple trashes
If you implement reversible deletion, you're faced with a fundamental problem: where does the deleted data go? Apple's simple implementation, where the files in question are moved to a .Trash directory, can still be surprising. For example, suppose my father has a USB flash drive he wants to save to, and is told that the drive is full, he might delete a bunch of files he no longer needs on that drive. But those files have actually just been moved to a .Trash directory on the same device (for speed; doing so means that you don't need to actually move the data, just fiddle the metadata to change the file's parent directory). Unfortunately, because there was already stuff in the .Trash corresponding to the machine's hard disk, the little icon in the Dock was already bulging full, so there's no obvious sign that this is what's happened. And it certainly wasn't my father's expectation: he knew he was deleting this stuff to free up space, and the machine had just told him he couldn't save because the device was full, so "obviously" it knows too.
I'm not sure there is a good answer to this problem. Make deletion non-undoable and you upset a lot of people (not many long-time Unix users, but plenty of real people). I'd be interested to see how people got on with an implementation that copied deleted stuff to a .Trash in their home directory (or, for bonus points, a user-configurable device; people with NFS-mounted home directories, say, might prefer not to keep their trash there for quota reasons). To me, it wouldn't seem unreasonable to treat removable devices differently from non-removable devices. Having deletion undoable only on non-removable devices might be harder to explain than "you can always get files back from the trash", but it does make it easier to answer the question of what and where the trash actually is.
Maybe the interesting question long-term is whether, when all Macs have multiple drives (or partitions) and Time Machine on by default, the concept of trashes will disappear from Mac OS? Will users be expected to use Time Machine to recover their recently-deleted files? It might work better than the current situation, and it might force people who'd otherwise ignore backup to investigate Time Machine...
You can scroll without fiddling with the scroll bar
...Which brings me, in a way, to the question of Apple manuals. Software like iWork comes with pretty thorough printed manuals, and stand-alone copies of Mac OS come with little booklets of varying quality, pointing out some of the more important features without actually going into much detail. The machines themselves come with little booklets, but experience suggests real people don't even notice. They're small, plain, and colorless. They come hidden away in a box that contains a shiny new toy, and they're discarded with said box (or safely stowed away with the box, depending on the individual's personality).
On the one hand, it's understandable, because "how hard can it be?", "how different can it be from the last desktop/laptop I used?", and "all I want to do is watch videos of skateboarding dogs on the web, so anything that isn't obvious isn't worth me worrying about". The trouble is, every ten years or so, something comes along that's both novel and useful, and these people miss out.
For example: you and I know that you can scroll by moving two fingers around on the touchpad. It's even mentioned on pages 21 and 26 of the "Everything Mac" booklet that comes with the MacBook, but the kind of person who reads that probably already knows about the feature from the research they did before buying the product. (The two-finger scrolling is mentioned on Apple's MacBook "Design" page on their web site.) Most users will only read documentation (or search the web) when they run into trouble. Searching the web is a great way to find solutions to problems you know you have, but it's a terrible way to learn about things you haven't imagined. Many people think "difficulty using scroll bars" is the price they pay for having a computer that warms their lap.
I personally think this two-finger scrolling thing is the biggest improvement between my PowerBook G4 of 2001, 666MHz PowerPC powerhouse that it was, and the 2007 MacBook Pro from my employer that I'm using these days. Sure, it's a much faster computer, but it's still a laptop with a low-resolution display and an unpleasant keyboard (funky backlighting notwithstanding) and I still find the least silly way of programming "on" it involves SSH.
While this post was sitting around as a draft, my mum mailed me to thank me for showing her two-finger scrolling. I'd never understood why, despite having more computers in the house than people, my parents still fought over the desktop. Turns out that a good-enough input device reduces demand for the desktop. (It would have been interesting to be able to run the opposite experiment, and give them a 17" MacBook Pro. I doubt it, for their skateboarding-dog-video purposes, or whatever it is old people do with computers.)
Is there a solution to this usability problem? I'm not sure. I'd have suggested a little peel-off sticker, but having just had the job of going round my parents' house removing those "remove before use" stickers from all their Apple power supplies, I'm not sure that's going to work. Maybe as part of the introductory video? It's long struck me as odd that such a thing exists and yet contains no useful content. Whereas the on-line iPhone introductory video was pretty awesome. Short and to the point, yet I came out of it feeling like an iPhone master, and can't claim to have learned much more from the (very good) iPhone manual.
iPhoto isn't part of Mac OS
iPhoto comes with every Mac, but it's actually part of iLife, not Mac OS. So if you do an "erase and install" against your son's advice, you'll lose iPhoto. And you can't just copy across the copy that's on your 10.3 Mac, because a 10.3-era iPhoto won't run on 10.5 (you won't be able to decipher the error, and there won't be much point your son explaining private frameworks, but you'll get the message that it's not going to work).
I think not including iPhoto is unnecessarily tight-fisted. Especially because it's not particularly great, and the fact that it's bundled with a bunch of junk (GarageBand, iDVD, iMovie, and iWeb) strengthens the gut feeling that it's a bit of a scam. I'd bundle iPhoto with Mac OS, unless the lawyers told me I couldn't, in which case I'd make it as cheap as possible. Aperture's the for-pay answer to that particular question. Not making iPhoto part of Mac OS just reduces Mac OS 10.5's already slightly questionable value to real people.
I was shocked to find I knew someone running 10.2 in 2007, but not all that shocked my dad was running 10.3 in the same year, and not at all shocked to find myself running 10.4 in 2008. (I'm running 10.5 on my desktop, but not on my laptop, because it came with 10.4 and by the time I realized I was allowed to run 10.5 on it, I didn't want to risk losing any of my configuration. Which you can take as my official verdict on 10.5: not yet interesting. Literally the only thing I'm missing is the ability to scroll a window without making it the foreground window. Which is the long-overdue fixing of a glaring bug rather than a feature. Bear in mind that I'm running 10.5 at home, so it's not like I've forgotten all its great advantages. There's no "there" there.)
CAPTCHA defeats old people
Finally, in this random collection of little things I noticed, CAPTCHAs don't just defeat bots: they defeat old people too. My mum had been complaining for a while that she sometimes couldn't send mail from one of the big-three webmail systems. What she hadn't managed to articulate until I actually watched her is that the reason is that she can't reliably solve CAPTCHAs. For one thing, her vision's not what it used to be. But I noticed also that because the CAPTCHAs in question don't actually mean anything (they're arbitrary alphanumeric strings) she was having great difficulty with, say, 'z', 'Z', '2', and '7' run into another letter. And I'll be completely honest with you: one of the ones she showed me that she was having trouble with, I couldn't solve either.
I'm not sure what the solution there is, but I do notice that I happen not to use anything that uses CAPTCHAs. I can't remember the last one I saw myself.