There were large numbers of PalmOS ‘apps’ available from online stores years ago, and I seem to recall they could even be downloaded direct to the later devices (e.g. the Treo). So what’s really different about Apple’s store? Well, in addition to better PR, Apple has:

1. a much more capable platform (better graphics, location services, motion sensors, etc.). there are whole classes of applications that just weren’t practical on a handheld in Palm’s heyday.

2. an already established ‘store’, with billing arrangements already set up for many customers.

3. much lower prices (PalmOS apps tended to be priced like desktop software, i.e. $20-$40, not $0-$10).

4. less fragmented – there’s one store, rather than several.

To me, it looks as though the two main improvements are pricing, and an easier, more familiar buying experience (leveraging the existing iTMS). But those relatively incremental improvements have made a huge difference in customer acceptance (just as the iPod improved on earlier MP3 players…).

There’s also the black art of getting developer mindshare, in which Apple’s had good luck (or good timing). The new Palm WebOS doesn’t seem to be taking off in this respect (so far), although Android may be doing so. That’s tough to replicate, whereas pricing and ease-of-purchase are relatively easy (although lots of companies still mess up in those areas).