There are three multimedia formats which I detest: Apple's quicktime video (mov), RealMedia (rm, ram, a bunch of others), and Microsoft's "MPEG4" format (almost exclusively distributed in the WMV container format).
There are several reasons for this:
1: They are closed proprietary formats. This means that officially they can only be played with one player (provided by the owner of the format). There exist other players which also can play them, but only using third-party hacks (which might even border the illegal in some countries).
2: All the official players for these formats are horrible. They are all extremely bloated (especially MS has made a really big effort to make their player overly bloated during the last years), they all have flashy but very user-unfriendly GUIs, and they are closed, proprietary and very platform-specific. Basically you would need three different players to play video files, and all these players suck.
3: The owners of the formats have been, and may in fact still be overly protective about their format. Especially Microsoft was quite infamous some years ago because of its bullying of third-party programs in order to make them drop support of the WMV format (VirtualDub might be the most prominent case).
4: Except for Apple's MOV format, the image quality of the other two formats is very low compared to free and open alternatives such as XviD. Compared to the latest MPEG-4 improvement, the H.264 format (also known as MPEG-4 Part 10), they are completely subpar. (With H.264 a video can be encoded to half the size of the equivalent WMV or RM with the end result having a much better image quality.) Apple's MOV implements H.264, which is about the only positive thing about that format.
5: These companies have used their marketing power to sneakily force their formats to become popular. Fortunately RealMedia has lost most of its popularity (it was way too popular in the past), but MOV and especially WMV are way too popular nowadays. This only causes the net to be full of crappy-quality videos.
6: And related to that: For some reason the net is really full of really crappy-quality postal stamp-sized WMV files. I don't know exactly how faithful Microsoft's implementation of the MPEG4 standard is to the actual standard, but if it works even slightly as it should, then people are encoding them in the wrong way. They think that making the image size smaller will result in a smaller video file and thus they scale the video to a ridiculously small size and encode to a ridiculously small bitrate, which results in a horrible video. Unless Microsoft's implementation is really crappy, this is the wrong way to do: With MPEG4 it's not the image size which is important but the bitrate. In fact, using a higher-resolution image with the same bitrate might in some cases improve the quality of the video.