1 Information overload
Wikipedia is full of useful information but as a the brain has a certain capacity for information absorption information overload can easily happen. Once the brain gets tired the site can seem boring.
2 Very long pages
Wikipedia has considerably longer pages than most sites. The fact that the the articles can be very long makes the brain anticipate a great amount of effort even before a person starts reading.
3 The style of writing is overly professional
The style of writing , which is considered very professional, can make the person believe that he is reading an academic book or a scientific paper. This style of writing can be considered boring by some people.
4 Many complex terms
Wikipedia is full of complex terms. The more the person can't recognize certain terms the more his brain has to work harder to understand the information and the more boring the page appears to be. (See Why reading makes you sleepy?)
5 Little pictures
Wikipedia is composed mainly of text and sometimes supporting pictures are added. The brain does more effort to understand text than the effort it does to recognize pictures. The low number of images makes the site harder to read.
6 Few white spaces
The absence of large wide spaces inside the pages of Wikipedia makes the text harder to read for the brain. A person can easily get bored of the site as his brain becomes tired.
7 The design looks so serious
The design of Wikipedia was made to look too serious to match the theme of the site. While the serious design helps the Wikipedia brand by making it consistent still it can make the pages seem boring to some readers.
8 It's associated with homework
Many people use Wikipedia for homework research. This fact makes the site easily remind those people of studying or obligations. This factor can make the site seem boring to those people.
9 Too many internal links
The fact that pages are strongly interlinked can make it hard for a person to understand one page without going through many other pages. The endless search for information can make people bored. (See Why Wikipedia is losing traffic from Google?)