Could you elaborate a little bit more on what you mean by less rigid boundary?
Sure! I can't remember where I first read the idea of "clear divisions, fuzzy boundaries", but it goes a little something like this:
You want to* have the different sections of your songs to be clearly defined from one another, so that someone could start listening at a random spot in the song and know where they are without any extra information. At the same time, you don't want to* have it feel like the song is just sort of going from section to section - "Okay we're in the intro. Okay now we're in the verse. Okay now we're in the chorus. Okay now we're in the verse. Etc., Etc." So like the first point you said about transitions, sounds don't always need to stop when you change sections. Conversely, sounds can be introduced before a section starts. A melody can start two beats before the new section starts, or switch from a synthesizer to a violin when you go from the chorus to the breakdown. Delay and/or reverb tails, reverses, and things like that help as well.
*: Standard songwriting conventions that can and should be ignored when it adds to the song creatively. e.g. a huge energy-filled dubstep buildup that suddenly cuts out so a vocal sample can play before the drop enters. The boundaries couldn't be more clearly defined, but that adds to the impact of the next section and just feels good.