These are all good thoughts. Obviously, different people take away different things from the Marathon series, which is why it has such broad appeal. Personally, I enjoy the Marathon series because it does such an outstanding job of mixing plot line, visual candy and action. It is engrossing in all three respects. My personal favorite is M2 because it tells a very well integrated story that is easy to follow, it is visually stunning and enjoyable to play. I agree that the MI textures are better, if not quite as intriguing as some of those in M2, and I, too, have played M2 using the MI texture set (I still prefer the M2 S'Pht doors, however - they appear alien and ancient).
My main gripe with MI is that the story is just too complex and poorly presented. I have no objection to nonlinear plot lines and, indeed, found Rubicon exceptionally well done in this regard. I never really understood the use of multiple universes in MI until I came here, however. The terminal text in MI is just too cryptic. It's a good story, but there's too much to read between the lines and one never knows why they are doing what they are doing. My first time through, I couldn't understand why in the world I would actually support the Pfhor, work for Tyco and fight the BOB's. The final insult is what I believe to be a very weak ending. I understand now why Durandal would have said that there was no evidence of the W'rkncacnter, as there wasn't in this final universe, but this didn't exactly prove satisfying.
Now, as to what constitutes an adult level . . . some of the people discussing this aren't even or are barely adults, themselves ;) . At least chronologically, I'm probably the most qualified to answer this. Let's put it this way - I started playing with computers when I was 16 - with a computer that took up a large room and used punch cards. To me, MI seems the most juvenile of the Trilogy for the reasons already stated. In fact, MI very much has the feel of a rebellious adolescent - sorry if I'm stepping on toes ;) . M2 will always in my mind be the most mature of the scenarios - it is certainly the most polished. M1 will always occupy a special place in my heart, however, as the most intriguing with an edge-of-the-seat plot line and a fear factor that has never been equaled except perhaps in M Evil and, to some extent, Red and Rubicon.
