There's an interesting tidbit in a Game Guru column by Nate Birkholz over at MacCentral today, a reaction to all this Doom 3 controversy, and about games that imitate other games. Marathon, however, is specificially listed as a game that while also a first-person shooter that came out after the downloadable Doom but before the shrinkwrapped Doom 2, was innovative and new:

Now don't get me wrong -- not every first person shooter was a copycat. Marathon, for example, actually came out a few months before Doom 2, and besides the far more compelling storyline, Marathon had some spectacular technology behind it, with the ability to look up and down, rooms above other rooms, and let's not forget voice communication in multiplay, a first for the computer games world.

And it also appears that Nate couldn't resist basking in a little Marathon reminiscence himself, as the postscript to his column indicates:

Note: Nate wishes to sincerely thank Doug Zartman of Bungie for actually taking his phone call, much less refreshing Nate's memory on the Marathon development and release timeline. Nate also wishes to apologize for making Doug say, They're everywhere!

They are indeed everywhere. The column is well worth a read, go and take a look.