True that winter in qatar is still not as people would expect but i don't think the temperature will be that high plus most of the players are professionals i don't think this will bother them much i think the high quality of the stadiums and infrastructure in QATAR will make them forget about it, i still think this will be the best world cup in terms of organization.
Maybe it will still affect how they play but most of the athlete that I know since they are competing outside their country, they will go to that country a month or 2 so that they can adjust to the weather and their environment especially if the country they compete is very different to their home. If I am not mistaken conditioning is their term every time they flew away so they can adjust.