Tiger to Leopard (Is it like XP to Vista)?

I'm not so sure about XP to Vista, but I would say that Leopard is smaller. Leopard is more like Windows XP SP2, but a little bigger. See, Leopard is essentially an update to Mac OS 10. Leopard is version 10.5. Apple is just very good at making people pay for upgrades. Heck, they've hyped it up to a whole new operating system.

So too is Vista an update to XP. Each 10.x milestone upgrade of OS X is substantial. They are not equivalent to service packs but are fully paid upgrades that have new features and often fundamental changes in them. OS X is pretty mature at this point, so the amount of change in each major release will be smaller - but the major releases will still be substantial. It's safe to say that 10.5 won't be as different from 10.4 as Vista is from XP, but then again opinions differ on the Windows side: Some people think Vista is a big change from XP, while others view it as mostly a face lift; rating OSs in this fashion can be highly subjective.

It's hard to decide how important a new major release will be compared to those that preceded it simply by comparing features on a piece of paper. In my mind, here are the most important major releases of OS X (in terms of changes from the previous version) thus far (from greatest to least): 10.2, 10.1, 10.3, 10.4. Where 10.5 will fit in that ranking I won't really know until I get to evaluate it myself.

Vista is about as close to a complete rewrite as MS will ever attempt on Windows...

Leopard is minor compared to Vista. Leopard is more like a service pack in the Windows world...

Here's the difference, going from Window 5.0 to Windows 6.0.
Leopard is 10.4 to 10.5.

Just because it has new features, or Apple makes you pay for it, doesn't mean it's not just a service pack

An OS change is an "upgrade" IMO if the new version is a genuine improvement, meaning that it makes life easier for you, or gives you new capabilities you didn't have before, or simplifies previously complex tasks. It's questionable whether Vista accomplishes much of that over XP. Change, for its own sake, isn't necessarily good, and change that makes life harder for you is most definitely bad.

That said, I haven't looked much yet into what Leopard will do for me or what changes there are as compared to Tiger, so I can't really comment. However, I'm hoping that the changes in Leopard will be genuine improvements. If so, then perhaps you could regard Leopard as a bigger (better?) "upgrade" than Vista, regardless of how much code is different or how much time/effort was spent on its development.

Tiger to Leopard will be smaller for several reasons: first of all, Apple has introduced most of Vista's revolutionary feature before it was even released, most notably a sound security concept, a state-of-the-art graphics foundation and all the Core technologies. Apple is doing a lot more fine-tuning now (e. g. interface improvements, architectural improvements) whereas Microsoft was trying to catch up and `make Windows safer'.

Vista had to catch up, and even though in some respects, it may look more powerful on paper, features are only important if you can make good use of them. Keep in mind that Vista hasn't been very successful. Microsoft doesn't release details on its sales, but the industry expects that the number of genuine Vista sales to be very bad (i.e. the number of copies of Vista that weren't bundled to a new computer). Dell has even reintroduced the option to get your favorite computer with a copy of XP instead of Vista. Companies are waiting as well until they adopt Vista on a grand scale. Leopard has created a big stir already and even though we obviously cannot say whether it'll sell well, Tiger has done very well and most Macs run 10.4 these days.

I wouldn't worry that Apple's offerings are outdated.