Howdy,
here are my thoughts for your consideration...
on one hand, it is a good thing to share one's beliefs (whatever they may be) with others. also an argument could be made that for those who need help in overcoming their fears about speaking to strangers or learning to "discuss" and justify their position (something that one must learn to do in the real world and in most businesses) being made or pressured to do field service results in a good end. for others the formal ministry helps them to become better speakers and presenters even if they are naturally gregarious or assertive.
it goes without saying that the Organization's Publishing Corporation Paradigm of the 20th Century is served well by having free in-person distribution of the endless stream of publications - so if one accepts the value of such publications then the door-to-door work furthers this
on the other hand, the stated goal of field service is not the mere distribution of literature or merely presenting the "Good News" but is supposedly to create bible studies which result in dedicated persons (converts). Just looking at that fact alone, one must acknowledge that the organized field service is a dismal failure and not very effective in meeting that goal.
additionally, as has been mentioned, some take issue with the fact that the Society attempts to find scriptural support in the actual "house to house" or "door to door" method when what actually occurred by Jesus, the apostles and the 1st Century church was a mixture of public lecturing and private teaching and never did these ones engage in anything resembling today's Witnessing house to house. Frankly opposers make too much of this point but perhaps it is justifiable given how much the Society touts the formal ministry.
The larger concern is that the ministry is considered integral to being a Witness, and while technically no quota is required, in order to be active, one must submit a formal report of at least their informal witnessing and for anyone wishing to receive "privilges" or reach positions of authority engaging in the ministry is required. Obviously, any type of "requirement" detracts from what should be a voluntary expression of the heart and a natural willingness to share the truth with others.
It is my belief that as the Organization shifts away from the Publishing Corporation model that the organized ministry will fade as feature of the culture of JWs.
-Eduardo