Here's what Lonely Planet Bhutan doesn't tell you, and some points worth saying again:

1. You have to pay the government tarriff. The government of Bhutan is wary of letting masses of tourists run roughshod over their fragile culture, which is why they use economics to limit entry to only 7000 foreign tourists annually (although there is no limit per se). This is accomplished by requiring all travelers to pay a tarriff, around $200/day, for entry. The fee also includes services of guides, land transport, lodging, food, yaks, mules, and horses (when needed). Unless you have an Indian passport, you can't get around that requirement. They check when you arrive. Thorntree has stories of travelers walking into Bhutan from Tibet and getting deported.
   
   Unfortunately, some recent tour books (not LP) perpetuate the myth that it is required to have a government invitation to travel to Bhutan and that travel must occur in a group. While a visa is required, group travel is not. Vist the government of Bhutan if you need to prove it to yourself.
   
2. Book with a Bhutanese tour operator. You can't go wrong since all tour operators are government regulated. I used Etho Metho because I liked one of the tours on their website. Booking a tour to Bhutan is as easy as falling off a log once you find what you want to do. E-mail the Bhutanese contact to save them the 10% commission that would be paid to their American agent. (The difficulty of making a wire transfer to New York City is greatly exaggerated in Lonely Planet, provided you have a semi-competent banker.) Don't book through a non-Bhutanese group tour unless you're masochistic and like group tours. You're paying a needless markon to the US operator, and if you're an independent traveler, it's worth paying the $40FIT charge to avoid having a roommate and doing things on the group's schedule.
3. Go in February or the shoulder seasons. Feb is charged as if it were the off-season (save $35/day) but the skies are clear and you're more likely to get into monastaries and temples that the tour books say you will never get into (that is, if your guide is willing to ask... be nice to him). In February, it will be cold, especially at night, but you'll have the country to yourself. Lonely Planet overstates the snow risk in February.
4. Declare yourself vegetarian. Generally, the meat in Bhutan is of low quality and comes in strange cuts hauled from India on the back of trucks without chilling equipment. Especially avoid pork; it is often very fatty and stiff and is also most likely to mess up your stomach. The beef isn't much better (we all know that Indian beef is terrible) and the occasional fish is typically fried deeper than an elementary school fish stick. You'll eat well in Bhutan (especially if you like chili peppers), but you can't go wrong telling your tour operator before you arrive that you're a vegetarian.
5. Change plenty of $$ at the airport. Plan on $10/day (if you like buying incidentals). Bhutan isn't an expensive country (e.g. 31 US cents for a litre of bottled water), but you'll be inconvenienced if you run out of money anywhere outside Thimpu. Dollars are OK for larger purposes if you have exact change, and you can easily use up your leftover rupees from a trip to India while in Bhutan. It took me a half hour to change money at the Bank of Bhutan in Trongsa, partially because the teller couldn't recall how to write the transaction in the ledger (no computers). You can give whatever Nu are left over as a tip to your guide. Don't plan on using a credit card. If the authorization office isn't open or if the person answering the phone doesn't pick-up for whatever reason, your card is useless. Visa is most widely accepted.
6. Watch out for incidentals. Among the great Bhutanese annoyances are the $20 visa fee, the Nu.300 departure tax, and the extra charges for soda, fruit juice, and water at meals. In an ideal world, these items would be included in the daily tarriff rather than nickle-and-dimed out of you.
7. Bring a torch. Electricity is unreliable and hotels often don't have generators (but they do have candles in every room).
   
   Bring Dramamine or other anti-motion-sickness medication if you venture east of Thimpu. The east-west highway is in bad shape beyond Thimpu and is miserable east of Trongsa on the day-long drive from Punakha to Bumthang.
   
   Bring soap and shampoo as Bhutanese hotels sometimes don't provide these.
   
   Bring a 3-pin Indian converter, which is not standard on "global" converter sets. If you need a converter, you'll have to wait till you get to Thimpu, where there are a dozen or more electrical shops. Here is a Bhutanese outlet--check now if you have the right converter.
8. Don't let Druk Air hold your tickets. Druk Air held my tickets at Bangkok and lost them, refusing to issue me a boarding pass. Fortunately they found them before the flight departed. Also, Druk Air does enforce the 20kg baggage limit.
9. Don't give anything to beggars, urchins, or grifters. This is good advise even here in Chicago, but the government of Bhutan strongly discourages tourists from giving anything (money, gifts, etc.) to Bhutanese beggars, and the police will chastise you if they see you. Bhutanese are hard working and self reliant, but I did encounter some boys near Punakha who wanted rupees. Don't feel obligated to pay children if they give you permission to take their pictures, although some will ask you to post a copy of the pictures to them. I also encountered a grifter in Jakar who wanted me to give him USD for Peruvian money (which was probably fake). Before parting with anything more than a few dollars, talk to your guide. He can tell you if the price is reasonable. Bargaining is not common except in the weekend market, and prices are arrived at quickly.
10. Be respectful of the Bhutanese. They are really friendly people who are often too kind to tell you when you're making a faux pas. Walk clockwise around chortens, take off your shoes in areas of prayer, don't bring out your camera in temples, don't raise your voice, don't be offended by the phalluses and erotic scenes painted on monastary walls, and don't act like a jerk from Chicago. Bhutan is literally and culturally as far around the world as it's possible to get from Chicago, which is a prime reason I went.