It isn’t all that long ago I played the original Marathon on my MacBook via AlephOne. I wasn’t all that impressed then, and I’m not very impressed with it’s sequel (why do the sequel first?) on XBLA either.
There’s nothing actually wrong with it at all. It’s just that Marathon is, and always has been, a poor man’s Doom. I got hold of an old Mac about 7 years ago with Marathon on it, and I didn’t like it then, so it’s not like disliking it is a new thing.
To me, it looks like someone has taken the LegacyDoom engine for Doom, and reskined and rearranged some WADs, and ended up with this. It just lacks soul, or something.