There's an interesting article from the official Guild Wars website made by a long-time player.
Special note: Each State of the Game presents the opinions and insights of one game observer. These observations are personal in nature and do not reflect the opinions of ArenaNet. While ArenaNet does review each State of the Game article to assure that it offers content that is respectful of all players, we intend to allow our reporters the freedom to inject some personal opinion into descriptions of the current atmosphere of competitive play in Guild Wars, and to express views based on their experience and observation.
Midline shutdown, in general terms, means the removal or disruption of opposing defenses or offense. In a previous discussion, we discussed the importance of frontline coordination. However, without proper midline shutdown, even the best Warriors will fail to kill their targets. In the next few articles, we will focus on the midline and its shutdown role against a typical balanced team.
Read the article in it's full glory here.