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The ground conditions. Polarization of a Antena. A vertical placed antenna is capable of producing quite low take-off angles without being placed high above the ground. For example: a normal GPA is able to produce angle's in the order of 16 degrees when it is sitting at the ground, a horizontal placed dipole has to be at 9 meters to does the same thing. A horizontal dipole placed lower then 9 meters will have probably worse DX results as the vertical. As said earlier it is not for nothing that for low frequencies (below 27 mhz say 7/3,5mhz or evan 1,8mhz), HAMs very often use verticals for DXing. There are stories going around which say that vertical polarized antennas supposed to be worse for DXing cause they're radiation pattern sloops along with the earth.(it continues along the earth further then the horizon). Although I do not have any fundamental prove that this is not true. The opposite did not happen either. I think it is not true, I can not think of a reason why the radiation pattern would not take-off under a low angle. One bad aspect of verticals however is know, a vertical above bad ground (desert) can't fill the high expectations which we just gave it. Horizontal antennas often outperform verticals under these conditions. At bad ground it is a bad DX antenna the reasons for that are to far to go into at this site. An example was that a horizontal 1 element quad proved to be 1 to 2 S-units stronger towards Europe from 234 division (very bad ground). Above good ground this effect seems to be amplified just think of those /MM stat ions.. This advantage of a vertical antenna, just might be where the Quad has got its great reputation from. A quad has two times a quarter wave radiating section which is vertical polarized. In the old days on lower frequencies HAMs reported that the band seemed to be opening earlier on a quad then on a yagi, the band seemed to close a bit later as well. So when you can not place your favorite antenna high enough a vertical can be a solution for you if you want to work some DX. There are also disadvantages to vertical, It produces more noises...so the station you want to hear is often deeper away. A vertical is more sensitive to QRM at the neighbors Above 8 to 9 meters a horizontal antenna will produce a lower or the same take-off angel, thus better for "real" DX. A Beam polarized vertical will be influenced more by the mast then a horizontal beam, unless it is far enough away from this mast or the mast is made out of non-guiding material. Most are omni-directional so all stations are heard, this can be an advantage as well. It performs poor above "bad" ground like desert. Tnx M.r Dave 30DA016 x info. 73' s dx' man. 
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CQ11.