Actually, iTunes was based on Soundjam, which, especially for the time, was a very decent player. Apple bought it, and it became iTunes. Thus, Mac users did have experience with MP3 players prior to iTunes. There were others around pre-iTunes also, but I don't know anyone who continued to use them after iTunes was released.

iCal has many, many similarities to MeetingMaker, a great app for the Mac that has also been around for quite some time. Meeting Maker is, in fact, superior in many ways, as it handles booking rooms, proxy access to other users, etc.

Mail is based on Claris Mail and probably draws some from Entourage and Eudora as well.

Safari draws influences from Chimera -- but there are a number of other decent browsers that were out for the Mac, including Omni Web, Mozilla, Netscape, etc.

iMovie has many elements of Final Cut Pro. Besides that, it's considered better than any WIntel offerings out there by every review or comparison I've ever read (which is probably more than 50).

iPhoto is also considered better by reviewers, as is iDVD.

Even Office for the Mac is often considered better than Office for Windows.

So, in many cases, alternatives to iApps exist or were around prior to release of the Apple-developed offerings. Furthermore, the iApps usually win out in side-by-side comparisons with their Wintel counterparts, so any gushing over them is warranted.