Web icons should be used carefully due to the amount of cognitive load that they generate for users. In fact, an icon is something that users have to decode to properly understand their meaning and semantics. What happens when an icon is polysemic? For example, what does a star mean? Favorites, bookmarks, ratings, or what?
The point is: without a proper explicative text used together with icons, the overuse of web icons may be harmful for users. This is both an accessibility and usability issue. Either people with cognitive disabilities or normal users may encounter problems when dealing with icons. Decoding an icon is something that is often difficult, because you have to think and, as many of you know, one of the most famous usability mottos claims "Don't make me think" (Steve Krug). Further, people with memory impairments may also be affected by the use of icons.
In a nutshell, icons should be used for what they really are, that is, graphics, and not as a mean to convey extra meanings. If you want to use them properly, combine them with explicative text, because what is clear and evident for you may be confusing or obscure for others.