Why are viola players stereotypically considered inferior musicians by nearly the entire orchestra? Simple.
The viola is significantly larger than a violin. I have a fairly short arm, and play a full size violin (about as big as they get) which has a body length of approximately 14 inches (35.6 cm). Compare to my viola, which measures 15.5 inches (39.4 cm). Likewise, the circumference of the neck of my violin in first position is about 3 inches (7.6 cm), and the circumference of the neck of my viola in first position is about 4 inches (10.2 cm). The viola and the viola bow are also much heavier than a violin and violin bow.
How does this factor into anything? Playing in tune on a violin, for an experienced player, is fairly easy. You fingers learn the places on the neck where each note falls, and that's it. Despite the occasional strain, playing in nearly any position, on any string on the violin can be done without changing the position of the thumb. However, playing on the viola is different. The extra distance adds up in difficulty in playing. For example, a G played on the C string with the fourth finger is nearly impossible to play in first position, unless you move the thumb forward to play the G, then back if you need to play for example, a D with the first finger.
Due to the size of the instrument, intonation is one of the biggest obstacles of the violist, compared to being simply an inconvenience for nearly any other instrumentalist. In spite of the inferior sound quality of an open string, for example, many players will use an open string instead of using the 4th finger: this isn't just the lazy shortcut for a poor musician, this is a neccessity for playing in tune. Many violists go so far as to tune the C string slightly higher than the actual pitch, which compensates for two things: firstly, the C string's tendency to stretch and fall in pitch faster than other strings, and also, so that playing in tune will exert less stress on the player's hand.
In addition to the issue of intonation, a high quality of tone is more difficult to achieve on a viola than on other instruments. On all stringed instruments, to prevent the "crunching" sound that plagues beginners, a perfect ratio of pressure and speed of bowing must be used by the performer. Too much pressure and too little speed equal a crunchy, displeasing sound. However, too little pressure (with, God forbid, too much speed), generates a thin, washed out sound, with no real body. On a viola, the window of opportunity, so to speak, is minute. Play with the same pressure and speed as you would on a violin, and your tone will be mediocre. Too much pressure, and once again, you have now entered crunch city. Factor in the added weight of a viola bow, and the sheer force needed to increase the pressure and speed to acceptable levels, and you have yourself one tired player.
This writeup does focuson comparison between a violinist and a violist. I chose to do this because the two instruments are so similar, and the source of many of the stereotypes regarding violas come about because many people don't understand the differences the between the violin and viola beyond the most fundamental.