While there is more and more information out there about Bhutan, when I was planning I found much of it was out of date as things change quickly. I also had some questions that weren't really answered until we arrived in Bhutan! So here are some things that would have been helpful to know before our trip.
1. Airlines may not have transfer agreements with Drukair in Bangkok. What does this mean? It means you can't go through the transfer counter in the airport in Bangkok, and will have to completely exit, including through customs, and then completely re-enter through security (including through customs). This is the case regardless if you have checked luggage or not (we did not). It also is the case regardless of whether or not you have a boarding pass (we had ours). Thai Airways does not have an agreement with Drukair, so build time into your itinerary if you have connecting flights and there isn't an existing agreement.
2. When Drukair says flights may leave early, they mean it. Be on time (meaning arrive 2 hours early as recommended) to the airport. I know Paro is a tiny airport, but when they want to get the plane out, they leave. When you see the maneuvers they have to perform to get out of the valley, you'll understand why–just go with it and be thankful that unlike airlines most places, they not only have the flexibility to leave when they essentially please, they will do so to avoid cancellations.
3. Be prepared for bad roads if you are going anywhere beyond Paro and Thimpu. I have an entire post on motion sickness, but if you are at all prone, make arrangements ahead of time, as the roads get bad quickly beyond Paro and Thimpu. They are unpaved, with huge elevation drops and gains (like 5,000 feet), and equally huge potholes. When it rains and dries, they get ruts. There are tight switchbacks. You've been warned.
4. While it cools off at night, days are hot. In September, plan to either wash your clothes and hang dry them (I bring powdered detergent), have them washed, or carry enough that you don't have to re-wear things. You will not want to re-wear things if you are hiking. We were absolutely drenched from head to toe in Punakha, and not much better off in Paro. It does get into the high 40's and low 50's at night, but by 9am it was close to 80 already.
5. Being in shape makes hiking much more pleasant. We saw some visitors who were really, really struggling. Particularly at the Tiger's Nest, but also on a shorter hike (albeit, mostly uphill) in Punakha. Do yourself a favor and do some walking, hiking, and aerobic exercise before you go. While everyone traveling from lower elevations will notice the change in altitude, being in shape helps. You don't want to be panting and drenched in sweat 15 minutes in to a 2.5 hour hike.
6. Hiking boots are not necessary unless trekking. There seem to be lots of questions out there about whether you need hiking boots for the Tiger's Nest or other day hikes. No. They are not. DH and I both had lightweight trail running shoes and were more than comfortable. There are not areas of loose rock or boulders, it's a trail and then mostly paved stairs. Hiking boots are truly over the top (and annoying to carry) unless you really need ankle support for existing issues. Now if you are trekking, this advice is not for you.
7. Everything really is included (except drinks, tips, and souvenirs). There were not any surprises; admissions, fees, etc are included in your package price. However, alcoholic and non-alcoholic beverages are not included, though all water is–even when ordered at lunch, etc. Souvenirs obviously aren't included. If you desire, you can leave a small amount (20 ngultrum or similar, approximately $.50) in temples as an offering. We never felt pressured to do so, and sometimes did and sometimes didn't.
8. Tipping your guide and driver is customary. I am a big proponent of tipping on the level of services received. The general guidelines that seem to be pretty universal, which we followed, were $8-$10 per day, per person for the guide and $5-$8 per day, per person for the driver. This can add a significant chunk of change to your budget, so be aware and be prepared. We carried, and paid them, in US dollars as this was easiest.