I've been 1K almost every year since the program began, spent $50K on 200K United Metal BIS last year, and was not offered the challenge.

Guessing the 25cpm is what's doing you in. This is an interesting data point.

How much of that was in business class? Makes a huge difference.

I had 200K+ BIS year couple years ago with mid 40s spend and did not make it.

It makes no difference at all nowadays. I qualified my first year with 1 paid segment in an international premium cabin (very heavily discounted GF).

At least 2 coworkers have made it flying transcons week in/week out in all sorts of economy fares.

The only consistent metric was >$50K spend.

May be more then just a total $$$$ threshold, based on earlier discussions there seems to be a cpm factor, some have made GS with closer to $40K spend but average 40 cpm. And some with $50K spend but with 25 or lower cpm, have no. So while all fares do seem to count now, a cpm-like threshold may have been added.

None - I buy coach. Lots of M fares and a few B.

Agreed. Seems like there's some average CPM that can get you there by spending less than expected, but that there is also a spend number ($50K?) that gets you in regardless of CPM (I know of at least 2 examples of <18 CPM but with just over $50K PQD).

About a week before they responded.

Mine took a little longer, almost 2 weeks, back in jan.
Just crossed the 15k yesterday, hopefully will see the GS reflected soon.
My last year stats, 103k BIS and abt 38k spent

Here is the rejection letter my dad got when he requested GS reconsideration:

We appreciate your commitment to United in 2013, and thank you for making us your choice of airline so often throughout the year. We've carefully reviewed your request to be reconsidered for Global Services status. Unfortunately, based upon your flight activity, we are unable to extend a Global Services invitation for 2014.

At year end, when determining annual invitations to Global Services, we evaluate multiple factors, the most critical of which is your flight activity relative to other members. In order to provide the highest possible level of service, we limit the number of invitations to maintain its exclusivity. We typically invite those who have achieved very high levels of revenue while flying first and business class tickets on United, United Express and Copa Airlines operated flights over recent consecutive years.

We encourage you to continue flying toward 2015 Global Services status, and thank you again for your exceptional loyalty.

emphasis added by me.
it seems that they don't evaluate how much you individually may have spent or flown, but your stats compared to others. which others it compares you to though is not specific.
is it me vs. others who use a diners club card, me vs. other people based in chicago, etc.

But yes, there's evidence the standards vary by home hub.

did your status change to GS yet?
 

I ask because I just crossed the 15k mark the other day as well...

Can any one please confirm that the challenge letter was only sent to previously GS that got down graded to 1k, I just got off the phone with MP and that's what they told me not sure if that's correct

My MP account received a PQD/activity update on 5 Mar that put me over the $15K challenge threshold. I then sent a notification email to both MP and GS. Online status updated to GS on 7 Mar. Awaiting credentials.

Were you down graded from GS before or this is your first time ?