I'd say that the main difference (for webnovels) is the media.
For example, I used to write for paper (nopes, never got published), and now I'm writing for the laptop screen.
However, a lot of webnovels are made for the smart phone screen.
Now a pleasant read is usually described in terms of language, textual layout and the big three, plot, character and setting. The problem is that the standard book-page more often than not is still taken for granted. Reading on a computer is close enough to the physical book, so the differences don't stand out as much.
What differences? Well, there's another factor as well. Just glancing at a page should be pleasant as well. Most of us notice different fonts and size of fonts and are aware how those make an impact. Most of us notice, but are unaware, when text is graphocally pleasant to look at no matter fonts and font-size.
why this text suddenly got hard to read.
Sometimes I pick up my phone and read my own works. In that format a lot of the words I use are simply too long, and my way of handling dialogue interspersed with character activities creates huge chunks of text which, quite frankly, are hard to read. It just looks that different from when I'm reading it (that would translate into when I'm typing and editing my own text) on a computer.
So I'd argue that an adaption of a webnovel into a physical book requires a lot more than line-editing. There's most likely a big need for structural editing as well.
And this is just from a graphical point of view. When you start adding how what I feel is horribly rushed pacing reads just fine on the small screen, then we're opening an entirely different can of worms altogether.