[fbags]

I bet all 1st gen Kindles can still buy all books from Amazon.

You lose this bet. There are quite a few Kindle books like this: http://cl.ly/image/2t2m2V3Y0513 that won't play on basic Kindle devices. I discovered this when I tried to read one of my books on my gen 1 Kindle.

There are also some unexpected restrictions on some titles, such as books that are viewable only on 1 or 2 of your devices, which is quite annoying to discover when you're not near one of the previous devices to deactivate it.

I can't comment on the functionality of iBooks on Gen1 iPads because I've never purchased an Apple iBook, but your guesses about Amazon are, sadly, wrong.

[comex]

I think the "available only on this devices" thing is about formatting, not DRM, although I could be wrong. (Of course, one might wonder why the original Kindle isn't being updated to support the new stuff...)

[fbags]

The point stands that I can't read those books on original Kindles at all. It's not like there's a "let me read the text, and don't show some graphics" option.

[SyneRyder]

"There are also some unexpected restrictions on some titles, such as books that are viewable only on 1 or 2 of your devices..."

You can check this when purchasing on Amazon: the small print you're looking for in the Product Details section is "Simultaneous Device Usage: Unlimited". If you don't see that, it means your Kindle book is restricted (I believe to 5 devices simultaneously, though perhaps it varies from book to book.)

[fbags]

It varies from book to book. Some of them are only 1 or 2 devices. The annoyance is that I inevitably forget which books are device limited and end up unable to open them from work / while traveling / etc.