Surprisingly many trekkers to Langtang report on catching AMS and related symptoms. In essence, the Langtang area is too easy for health... it is deceptively easy trekking getting you too high too fast. If in good shape you can actually reach Gosainkund and Laurebina La toward Helambu in one day's trek, but if not acclimatized beforehand that's guaranteed recipe for disaster.
Even walking into Langtang Valley it is easy to go too fast. You need to force yourself to stop - or walk real slow and take in the scenery and good "AMS-free" air on the way.
This stretch of road has never seen asphalt and might not see it in decades.... There is a huge landslide area that the road to cross, and negotiating this has led to many, many bus accidents. From spots along this road (also closer to Kathmandu) it is not infrequent that buses do not turn while roads to and the other way around. And it is a long tumbling way to the bottom of these valleys, especially in Rasuwa. Be careful....
The buses running the distance between Kathmandu and Syabrubesi keep falling off the road to the great anguish and regret of those involved.
It doesn't help to call the buses "Supersonic Express" when the buses run at an average of 12 km per hour and still keep tumbling into Trishuli River. This is one of the more risky public transportation stretches you can do in Nepal.
There is a part just above Saraswati Kund where the trail passes a very steep scree area and the snow sometimes stay long here. With time it may get frozen hard and sometimes turns into nearly pure ice, and there is no foothold. This is the most dangerous situation here; if you loose your foothold, start to slide and fall there is 2000 meters until you'll stop. Before you attempt to pass up to Gosainkuda in the cold season ask for local advice and/or be sufficiently equipped with ice equipment to make it across safly.