This is where my main disappointment was: there was no ONE leader throughout.
Jo in Thailand was great, and he also came to Laos with us, but we were accompanied by another local guide in Laos who was really not much use at all. It was quite strained with them defering to each other and made for a few awkward times. Then both of them left and we had yet another in Cambodia who was just OK: efficient in logistics but rather impersonal and not good people skills and didn't really gel with the guests. Then he left us and when we got back to Bangkok we were very much on our own, with just a rep. So essentially the trip felt like it ended before we left Siem Reap.
I would highly recommend having ONE leader throughout because the way it is structured now lends to a very disjointed trip, and unavoidable comparisons from one leader to the next: unfavourable in this case.
I found the weather really great in January - no warm clothes necessary. There are laundromats available in a number of places (Luang Prabang, Vang Vieng, Siem Reap) that are very cheap. For other times, bring easy-to-wash/dry stuff and do it yourself. Easily dries overnight. Travel light. No need for fancy clothes. I had an overhead locker size case and managed just fine (and was the envy of the group who struggled up stairs with heavy cases!)
A torch/flashlight is useful for walking back at night/caves etc
Hotels on the whole were good, with aircon if its hot. No really long days traveling; and stops are frequent. Loos are way better than I had imagined, though you do have to pay for some. But paper not always on hand.
Food is excellent - wide variety, and very reasonable. All hotels provided plenty of bottled water so that saves quite a bit.
I think a couple of things would greatly enhance this holiday, in addition to my comment above re having one tour leader.
1) make the trip a little longer and include Phonsavan to see the Plain of Jars.
2) do the included sight-seeing in Vang Vieng in the afternoon of the day we arrive (got there about lunchtime), stay one night then move on the next day to Vientianne and have 2 nights there: it is a beautiful city.
3) make the visit to Tonle Sap Lake stilt houses part of the itinerary: it was a highlight for us all
4) include an overnight in Batembang in Cambodia
5) where hotels have a very scenic setting eg Chiang Kong, Pakbeng and Vang Vieng, at least arrange river view rooms for us! Mine overlooked the carpark in 2 places, and was in the basement in the other.
6) don't leave Siem Reap at the crack of dawn on day 16! There are 5 flights a day out of SR to Bangkok so why not leave on the late evening flight? There is much more to see and do there, and it is a much nicer town. We all arrived back in BK mid-morning and felt very much like the trip was over and no one really wanted to do anything, or have a final group meal etc. Disappointing. If it can't be changed, why not have some included sight-seeing arranged for the final afternoon in BK and include a final dinner? At least the holiday would end on a high note.
I realise that some of these would add to the cost of the holiday, but all said and done, I don't think the additional days/cost would put potential travelers off: if you're spending this amount of money, a little more for much better itinerary and rooms would be money well spent.