It's not fair to criticise the art in CoH for not having loads of monsters, as they're not really in Alan Lee's style. He is known for being subtle, and often hides characters in the depths of his paintings so that you really need to look for them (kind of a Middle-earth "Where's Wally?" if you want to look at the fun side of this
). However Tolkien 'hid' characters himself in his own artwork, so Lee is following in a tradition. The landscape is always the central thing, bigger than the people who live in it, which brings across the feeling of 'epic' - note how Lee cleverly only shows us a part of Orthanc as if to emphasise just how monumental the tower is.
He is also a very quiet, modest man, and I think his art reflects his personality, including in the muted colours he chooses to make use of.