The author has less control of the font than in a printed page. The author has less control of the formatting than in the printed page. The resolution of the medium is less. Fine detailed fonts do not therefore render well. Readers tend to scan rather than read.
Scroll bars add a new factor.
Up until now, it has been difficult to make any serious attempt at laying out a page for easy reading. The author had very little control over either their fonts or the formatting. CSS is changing all that. For the first time it is possible, or rather it will be when CSS2 positioning is fully enabled in the major browsers, for the author to specify his page layout. Authors can now be reasonably certain that their pages will appear with the specified font, and with the specified formatting.