Leatherneck is essentially mando for those wanting to go Marines. Exemptions for MAGTF include injury/getting surgery, sports exemptions (i.e., championships during when Leatherneck is), and a few people who are "extremely qualified" for Marine Corps (including priors, but a lot of them still went to Leatherneck), but the total MAGTF numbers are very small.
I'm a firstie Marine select (rockin' the "Warrior Friday" attire as I type...), so I've got some decent gouge.
The number of Marine ascensions basically peaked with my class at ~272 (roughly 2/3 Ground contracts, 1/3 Air). The baseline number of Marines will likely decrease over the next few years.
For this year, I've heard different numbers as to how many people put Marines first, but they turned away roughly 30-50 people who had Marine Corps as their first choice, along with some guys who had it second after SEALs/EOD. This does not include people who were told after Leatherneck to not even bother putting Marines first since they wouldn't get it. There were probably a few of those per Leatherneck platoon, so tack on another 20-30 who wanted Marines but didn't get it.
What this means for future classes is that the requirements for even attending Leatherneck are increasing. The current 2/C (2013) have to have a 250+ PFT score and pass the O-course to even be considered to attend Leatherneck. This is because for my class and previous classes, anyone with a pulse could attend Leatherneck, and so there were 320+ people down at Quantico this summer, but there were a lot of people there who probably should not have been.
Traditional wisdom was that anyone even vaguely considering Marines should attend Leatherneck just in case, so there were a lot of people who wanted Navy Air who went to keep Marine Air an option, etc., but really had no desire to actually go Marines. Because there's now entry checkpoints, I'd imagine that'll change and people will have to make that call earlier.
How the actual Marine selection works is they take a bunch of Marines from around the Yard and sit them down to pick people. They review the packets of every single 1/C who put Marine Corps first on their preference sheet. Each company has a Marine officer assigned as their company "Marine Mentor," and the mentors are the ones who brief those mids to the selection board because they theoretically know the MIDN the best.
Probably the single biggest factor for actually getting Marine Corps was performance at Leatherneck, all other things being equal. A guy in my company was our Battalion Semper Fi (Marine interest group) representative, but got sunk by his performance at Leatherneck.
You are graded at Leatherneck on physical, academic, and leadership performance and are given, essentially, an A, B, or C: Recommended with enthusiasm (A), recommended with confidence (B), or recommended with reservations (C). Alternatively, you could get not recommended, period, but that's fairly uncommon and reserved for only the worst performers.
Aside from that, a good PFT and CFT score, passing O-course and E-course, and decent aptitude for commissioning grades throughout your time at USNA are big factors. As far as academics, they mostly care that you're going to graduate. For hopeful air contracts, ASTB (aviator/NFO aptitude tests) didn't seem to matter as long as you got the required scores: I think I got the minimum score on at least one section and was still selected.
Speaking generally: A 2.5+ GPA (pretty low), Enthusiasm/Confidence Recommendation from Leatherneck, Okay-to-good O/E-Course times, ~270+ PFT and 285+ CFT (it's easier) with no major conduct offenses and NO HONOR OFFENSES would be almost guaranteed to go Marine Corps. But, that was for my class: for 2013+ as the numbers allotted to us go down, that GPA/PFT/CFT will probably go up.