the big difference I see between the SEC and everyone else in depth and variety.
I think that's pretty much it, although I'd state it a little differently, in terms of tradition and basic program fundamentals, rather than the strength of teams right now. To me, it's about the number of elite programs in the conference. The SEC simply has more of them than the BigTen does, and that is unlikely to change.
The elite programs, by my definition, are not necessarily the ones that are really good right now. They're the ones that have the two basic fundamentals in place that are generally required to be consistently good.
1) Name brand (this is an amalgam of winning tradition/fan support/media interest)
2) Geographic proximity to great high school football talent
I would say the SEC has about 6 programs that are elite by this measure (say, Florida, Tennessee, Georgia, Alabama, LSU, Auburn), and maybe a couple more that could conceivably get there with some brand development. The BigTen has 3, maybe 4 (OSU, Michigan, PSU, maybe Nebraska) and maybe one or two more that could conceivably get there with some brand development.
Basically, the SEC has more programs that are realistically capable of competing for a national title on a regular basis, and that's a disparity that isn't likely to change. The perception of the disparity has certainly been increased (somewhat unreasonably, in my opinion) by the fact that 2 of the BigTen's 3 indisputably elite programs have been down for the better part of a decade. But one indication of the SEC's greater overall strength is that they can absorb a downturn in a couple of their elite programs (like UT, recently) without it turning the conference into a one trick pony.
That said, I think the BigTen can, should, and probably will be in the near future consistently among the strongest of the non-SEC conferences, according to the same analysis.