I've been to a hotel recently that was just newly built and which had the latest window technology for heat insulation. That's all nice and well but for mobile networks on the outside, such windows are almost impenetrable.
The path loss through these windows were 30 db, so even a base station pointing directly at the hotel having a great 3G signal outside the hotel (-60 dbm) dropped to still acceptable (-90 dbm) just inside the hotel to a complete "nothing" just a few meters inside. Without dedicated in-house coverage via leaky cables, femtos, repeaters, etc., hotel guests are having a hard time if they want to use the networks outside.
Interestingly enough, standing close to a window helps but standing to a wall is actually much much better, the path loss in my case was only 10 db instead of 30. Quite a counter intuitive thing to do.