(b) You can't make glossy marketing brochure type pages (although some would say this is an advantage); can't always make things look nice; can't use your choice of colours and fonts (although again, some people would prefer it if you couldn't influence their web browser's behaviour in that way); etc. (Choose any one.)
(c) Automatic processing of all kinds would not be able to extract the structural information, so you wouldn't get an appropriate window title or bookmark value, it wouldn't be indexed properly by search engines, and so forth.
Note: Some students noticed that since <a> and <img> are considered structural tags, this restriction would mean that you couldn't have hyperlinks or embedded images. This is not the sort of thing I was trying to get at in part 'c' of this question, but I had to admit that this was correct and should get full marks.