When I say boring, I am speaking comparatively, because the truth is that I find all forms of tennis exciting whether it is servebots or a grade 5 junior tournament qualifying. Having said that, of the three main surfaces clay is definitely the least entertaining.
The typical characteristics of a clay court are that it is slow and slippery with high bounces, and the ball grips the surface when it bounces. What this does is makes serves and shots with back spin both much less effective, leading to less free points on serve, less net approaches, longer points from deep behind the baseline, and ultimately more breaks of serve.
There is nothing wrong at all with longer rallies and less net approaches if you ask me, and even breaks themselves aren't a bad thing. However, the result of these things is that la terre battue produces the least competitive and therefore least entertaining matches of any surface.
Since almost every point is very similar the dirt in that a point stays neutral for a long time, what happens is that the better player wins the point more often than not. That means that the breaks that are happening more frequently are all going to the same player, which is the better player. Because of that there are more straight-set victories on clay than any other surface and less upsets, making it the most non-competitive and predictable of all the tournaments.
A while back, I wrote a piece on which set is the most important set. Out of the research I did for that article came other information about, which tournaments have the most lopsided results. I looked at the 2013 US Open and the first three grand slam events of the 2014 season on the men's side.
Among those four grand slams, Roland Garros, which is the only major on clay, had 76 straight-set matches. The 2013 US Open (hard) had 56 straight-set results, while the Australian Open (hard) and Wimbledon (grass) had 61 and 60 respectively.
One thing that does tend to happen on dirt is that players get worn out because of the long rallies, and many times give in to cramps. So the one way there can be upsets is when there is a drastic momentum swing, caused by fatigue. At Roland Garros in 2014 there were six comebacks from two sets down. The Australian Open had a surprisingly high number of five in 2014, but there were just two at Wimbledon and three at the US Open.
The ATP World Tour currently consists of just one grand slam on clay, while only three of the nine Masters Series events are on the brick dust. Plus, even the year-end championships in London are played on a hard court. Many of the best tennis players in the world right now grew up playing on dirt - Djokovic, Federer, Nadal, Del Potro, and Ferrer just to name a few. However, making the dirt a more common surface on tour would not be a good thing to do to bring in more fans to the sport.