There is the issue of internet forum based discussions being a little more feisty than most debates anyway but I think the main problem is that generally there is a huge gulf between what most atheists feel about theists and what an individual atheist feels about a particular line of argument.
There is a poster on this forum who insists on posting the most meaningless drivel on every thread on the subject of atheism and evolution without ever posting anything of worth, this particular poster gets quite a bit of stick for this. Does that mean that they deserve it for being a theist? No, clearly they get called an idiot for being incapable of rational argument. I personally will happily tell someone they are talking bollocks if that is what they are doing, but I fully respect peoples right to believe what they wish.
Theists and creationists have to expect to be called upon to back up their claims just as I would expect to be asked to back up my claims. My experience has been that generally, creationists fail to respond to such requests (unless you are counting 'bwahahahahaha' as a coherent response ) Based on that I find creationism (at least in the fundy YEC and ID brigade forms) to be a less than intellectual stimulating worldview and I have little time or patience for anyone who subscribes to such shallow egotistical nonsense as invariably the same people are usually incapable of adequately describing the theory they are arguing against.
However with all of that said I am fully aware that science is incapable of 'disproving' god and as such I have no argument other than Occam's razor against a god that kickstarted the big bang or abiogenesis, it's just a shame that the razor is rather unmerciful to supernatural creatures.
So I have no problem with creationists other than their propensity for talking rubbish on subjects they don't understand, other than that they're great!