I think this is partially because of the medium, and partially because of some of the posters!
Bulletin boards and blogs are often filled with emotional written statements. (Which is not a bad thing, btw.) Unfortunately, just words do not allow the heart of the person to come through. This partially creates this situation. In the edition of Finney's Revival Lecutures that I have there is a comment to the effect that in print Finney comes across as cold and stern, but in person his words would have been better understood because the hearers also would have felt his tender heart.
The only solution I have found to this is editing. A good, clear response will require a person to write, edit, write ,edit, etc etc until they have a clear and precise statement. Often times I will write my response in Notepad, then cut and past it into the response box. It is very important to me that what I say be clear, and above all, be free from sounding accusatory, scolding, bitter, or attacking. There may be times for that, but it is the exception, rather than the rule.
As far as the matter of the posters, I cannot talk for them. There is no accounting for personality. I think Keith Green's "For Prophets Only!" should be required listening/reading by everyone who thinks they are called of God. (Yes, it is on here, but its called his testimony.) A close friend of mine who studied at Last Days and WYAM told me that Keith Green was once pointed out to him as an example of how God can use even obnoxious people. I know that sounds like heresy, but I think Keith himself realized how much damage not acting in love could do.
From my perspective, there are many people who read Finney, Wesley, Ravenhill, etc etc and start to take their preaching and personality styles from those books. I did that myself in my early years in ministry. As a young preacher, it makes you SOUND like you have authority. But that's all it seems to be...sound.
I drove more people away than I drew in. It wasn't until I started seeing people as Jesus saw them...as PEOPLE (not as "sinners" who were somehow different from myself) that I started to have results. I also started to realize I didn't have to argue with anyone...the truth was the truth, so there was no argument.
Its pretty liberating to know you don't have to be negative, and that you don't have to point out people's sin to them. When the gospel is presented to them, they figure that out. Trust me, I've lit into people who were scripturally wrong before. But it wasn't because it was MY opinion...it was because it was unscriptural. And you can do that without being nasty too. :-)