I do not think that I can name all of the obstacles to reading. There are simply too many. I will list a bunch, but there will also be different obstacles to different readers. Those might be based on reading level or even reading location.
I know that for my own son (a kindergartner), he struggles with letter identification. For example, he often confuses "b" and "d." They look a lot alike, but they are very different letters in words. Similarly a young reader may struggle with the concept of reading left to right. Young readers can often struggle with identifying where the spaces are between words as well. Then the textlookslikeonegiant mess.
Another obstacle to reading might be a limited vocabulary. Again, my son struggles with reading because of this. He's great on books filled with words he knows, but some of the books we are getting lately are introducing words he has never read before. That slows him down because he has to sound it out. By the end of that kind of story, he's often so frustrated by his slow progress that he practically shuts down completely. That response could be true of any age group too. Even as an adult there are some books that I've come across that are so filled with giant technical terms that I spend more time figuring out meanings than I do reading the actual text. They are not fun to read. Things that are not fun often do not further motivate readers.
Outside distractions could be obstacles to reading as well. There could be noise distractions or people distractions. There might be other stuff on the reader's mind. The reader could be too tired to focus on the reading. A barrier could be a dislike of a certain genre or author.