Funny you should post this. I was just talking with a friend this morning (also a Beijing-based expat) who just returned from a 10-day trip to Bhutan with his wife and small group of 5 other expats. He said it was brilliant for both scenery and cultural/people aspects. Their group was all older (ages 45-60) professional types and "Old Asia Hands", none were backpackers/trekkers or even very athletic. In general, their group went in two vehicles (with required guide) on OK roads, but many of the monasteries and old fortresses had to be accessed by some relatively strenuous hikes, with thin air requiring one to go slowly. But generally, he said no problem and no oxygen needed. He also said most common trip length is 7 days, but he was glad they had 10 days (central and eastern part of Bhutan, they didn't get to western part). And one could fill up 14 days if you had them. He said a surprising number of people spoke decent English, as it's required from an early age in schools.
That said, Bhutan is hardly going to be a mass tourist destination, ever.
1) Bhutan strictly limits tourism and doesn't want large numbers. Airplane seats into the country on Druk Air (the primary way into the country) are limited. There is a land border open with India but it takes some effort to get there.
2) There is pretty much a set price, approx USD $200 per day which includes everything (a little bit lower per person if you have a small group, more if there are only 1-2 of you). This pricing weeds out most mainstream and budget tourists right there.
3) There aren't many accommodation facilities that have enough space for larger groups and especially outside the capital of Thimphu. Mostly small inns and guesthouses.
4) A group tour has to be set up using an approved Bhutan agency, and use of a guide is required. Even if you're a group of one. This puts off die-hard independent types and people who like to do things on the fly.
My friend is sending me information on the company/guide they used--which he highly recommended--and also on their exact itinerary. After my conversation with him, it's zoomed up the list of my Places to Go and possibly sometime in 2010. Apparently best times to go are around mid-March to mid-May, and then September-November. Summer not good due to monsoon and landslides, and winter may be too cold and snowy to enjoy or even to access (though I'm sending out inquiries about this to Bhutan shortly RE: February possibility).