But that doesn't really address Dr. Rush's point about ME1. Most of the UNC worlds are a crashing bore -- well, YMMV, so let's say "worse than the critical path content." Unless a player can't tell which content is which, I don't see how a casual player doesn't end up sticking with the high-value content if he finds the low-value content unattractive.
Your comment doesn't make sense to me. First time players really have no way of knowing where the "high-value content" is without going through the game once (which would mean they would be credited with a completion, right?). Some people I knew were really turned off by the mako... and it soon seemed to them that they encountered the mako everywhere, with the repetitive drop on each UNC world as well as at the start of Therum and soon after the start of Feros and Noveria. (if they made it to Virmire, they probably completed the game). Also, for the "casual" difficulty player, the combat mechanics got pretty repetitive after a short time since you could just basically enter the room and just stand there firing away endlessly. Also, the main missions had set pieces that were, in many ways, very similar to the UNC sets. For example, Therum's mines don't really look a whole lot different than any of the other UNC mines; and the docking bays at Feros and at Noveria look very much alike as well.
By comparison, ME2 offered a lot more variety in the look and feel of the set pieces for the various missions.
In addition, new conversations weren't triggered unless the player advanced to doing one of the main missions or one of the loyalty missions. That meant that first time players that felt they should talk to the crew after each and every UNC mission found themselves engaging in the same convos over and over again... which could also lead to them just getting bored, thinking the game was bugged, or just giving up and quitting the game.
As others have said, I think the fact that ME1 did not come out on the PS3 until after ME2 played a role. If they are still keeping track, I expect that the ME1 completion rates relative to ME2 will climb here prior to ME:A simply because ME1 is backwards compatible on the Xbox One and ME2 and ME3 aren't. Since ME:A is coming out on the Xbox One and not on the Xbox 360, I expect some people will play ME1 to get a taste of it before ME:A is released but won't be able to finish that Trilogy on the Xbox One.