It's just everything. The reliability of the OS, the hardware, the integration of the software.
I agree with most of what you said, but it's more incremental than that, as well. 10.5.8 on my upgraded G4
is almost perfectly stable except for the fact that some games that worked under 10.4 don't work right and Classic is gone (huge loss for using classic software and unnecessary, IMO as seeing 10.5 was going to be the last PPC OSX version, they
should have gone ahead and included it for those PPC machines so they'd have the best combination of features and software to finish out with.
I agree that 10.5 was unstable at first, but I LOVE the 'spacebar preview" feature it added, the 3D dock does look better (even though I now use the left side dock for space constraint reasons) and Spaces was a great feature addition and makes it much more usable, IMO. I don't use Time Machine so I couldn't care less there (I use CCC for perfect bootable backups which has saved me a massive headache on at least one occasion where my PowerMac got a corruption from a bad software update and it also made it simple to upgrade my internal hard drive on my MBP since I could boot from an external and then copy back to the new internal).
In fact, the ONLY thing that has made my PowerMac unstable with 10.5.8 is certain iTunes versions over the past couple of years. iTunes itself would be unstable on PPC and it would take down the entire machine with it with those versions, usually within 24 hours of starting it (I use it as a server for a whole house system so it's on 24/7). I had to revert to the previous stable version when this happened (10.6.3 is on there right now and it's rock solid; Apple usually fixed the problems in the next version when I reported it. Obviously, they didn't test PPC versions as well as Intel in recent years). Now THAT shouldn't happen, but it's not like I've never had 10.4 come up with a kernel panic either. In fact, I'd say other than those times, 10.5.8 is more stable than 10.4 (which itself went through a LOT of updates so I can't say it was stable in every single update either).
I don't think I've booted back into 10.4 more than 3-4 times in the past 3 years (to play an old game, really) and NONE in the past year. I can still boot OS9 on this machine (although sound usually doesn't work since I upgraded the CPU to 1.8GHz and I'm not sure what the issue is there and there's no accelerated driver for my ATI 9800 Pro anyway, so it's mostly useless for gaming regardless at this point and that's the only real reason to boot OS9 anyway since most other OS9 software works fine in Classic mode in 10.4). From that I learned it's best to buy Windows games as they work longer and are better supported and generally run faster anyway.
A PowerMac G5 in 2005, running Tiger and iLife 05 was literally bug free.
Other than the occasional glitched iTunes (which was glitched in Tiger too until they stopped supporting it some time ago), my PowerMac G4 running Leopard is STILL "bug free" (as in it's normally up for MONTHS at a time without a reboot). In fact, it's been up now for 41 days and counting and the only reason it was shut down at that time was because I was on a trip for 6 weeks and saw no reason to waste power during that time. In fact, the only impediment to continuing to use it as a server is that iTunes is no longer being updated for it as of the 10.7 release. As long as I don't buy any newer iOS hardware, I still have no reason to upgrade. But it's getting long in the tooth in terms of browsing web sites (even with TenFourFox to keep it running) since the Web is using more and more GARBAGE on their sites that need more CPU power (ridiculous in itself IMO since I don't feel the Web is one-bit better for having loads of Twitter and Facebook icons on all the web pages and hide-away data that doesn't need to be loaded until it's asked for, loads of garbage advertising tricks, etc. etc.)
There were no problems. I remember it clearly, and I confirm it every time I work on a pre-2006 tower. They run their native OS's smoother and more glitch free than the current machines run their native OS's.
I don't run my MBP 24/7, but I did use it to make an album with Logic Pro over 2.5 years and considering I normally put it to sleep in-between sessions and not shut it off or rebooted, I think it held up amazingly stable. The amount of memory thrashing used in that program is off the scale and while I did have to quit and restart logic once in awhile (never failed to save even when it crashed, though so I never lost recorded data, which itself is amazing and speaks volumes about how professional such a complex "all-in-one" recording studio/LAW Logic is, but other than a few times for the odd software update for 10.6, I never had to reboot. That sounds pretty freaking solid to me and that MBP came with 10.5 and Leopard was pretty solid it on it as well. Admittedly, 10.6 was not very stable until 10.6.3, but I didn't upgrade until then anyway (I wait until the reports back are solid before upgrading machines I actually use for utility).
iMovie 06' is a perfect example of when the downhill started. They added a bunch of features that didn't add anything useful to the application, but all of a sudden it was taking twice as long to export videos or to burn DVDs. The program was just slowed by 50%... apparently for no reason. I don't
I agree that the "faster faster" aspect of OSX stopped with Tiger. Leopard was definitely slower than Tiger on PPC and Snow Leopard was slightly slower here on my MBP according to tests than Leopard (something I always found odd for an "optimized" version of Leopard), but that doesn't mean it was unstable by its final version.
I never tried Lion (didn't want to lose Rosetta) and it got so many bad reviews, I didn't even want to try it. Mountain Lion sounds like something I would like to try, but unless I can virtualize 10.6 to get Rosetta for a few apps, I'm not gung-ho to lose software usage when I don't 'need' any of those "features" (frankly, Mission Control sounds like useless crap and Spaces was destroyed in its most useful form and I'm a mouse guy, not a trackpad one, so any changes there mean squat to me).
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Aside from performance, it give the opportunity to have the handling of each type of data optimized instead of a kitchen sink approach that wasn't ever originally intended for many of the functions that are now shoehorned in.
Maybe it was not "originally intended", but obviously since AppleTV came out and to an even greater extent, the iPhone and all the new iOS devices, it's now indispensable and frankly, it would be a disaster to remove the "kitchen sink" functionality at this point. That is why I say if someone "just" wants a music player, look elsewhere. Better yet, write your own. iTunes NEEDS to be a full-fledged media handler at this stage or the entire iOS world just falls apart. Microsoft has their own "do-all" media handler as well (Media Player). The world has changed. Devices do more than just play MP3s today. They're a lot more powerful. Get used to it.
I've never had my iPod Touch crash (either one of them) so I don't see what the big deal is about having "music only" apps. ITunes runs here 24/7 on a PowerMac G4 and typically runs for MONTHS without issue and that machine only has 1.5GB of ram and still runs browsers, e-mail and other programs without any issue while iTunes continues to serve my entire house in the background. I've never had iTunes skip for sound when playing music either, not even when browsing and that's a single CPU G4.