1. It's possible to get more tired playing the viola, as it is larger and heavier than the violin. Luckily I haven't had this problem yet and the viola is still more comfortable for me, but I am aware that it is common for this to happen, and maybe once I'm doing more serious concerts with full-length symphonies I'll experience this too.
2. Opposite desk sides...this means that the "outside" is the "inside" and vice versa. So, one has to remember whether they're the one turning the pages, and who's playing top/bottom and generally get used to the opposite setup!
3. Different orchestral positions - while the first violins never move (that I have seen, anyway) from their position in the front at stage right, the violas often play on the outside stage left, and also often play between the second violins and the cellos, in the middle.
4. Violas do actually have the ability to make more sound than violins. This is why there are fewer violas in the section. It takes more effort to get a good sound out of a viola than a violin, and this extra effort translates to a louder sound. (Whether that sound projects as well is another story, however.)
5. Sometimes, the viola part may not make sense. Within itself, that is. Unlike the first violins who almost never have a line that doesn't musically make sense, the violas seem to often have combinations of notes that seem musically illogical, making the part harder to anticipate. In context the viola part (usually) makes perfect sense, so we violas have to get used to interpreting the music as more of a whole than the violins, who can get away with knowing it on a more superficial level and living in their own, self-absorbed world.
6. The violins sometimes sound pretty...interesting. As everyone knows, they're the ones with all the high stuff and when first reading/rehearsing these parts, it can sound rather...well, awful. More so than one can appreciate when playing as one of them! Luckily they also improve quickly, but hearing a first run-through brought me thoughts of "That's what we sound like!? Glad I'm not doing that anymore!"
7. Finally, even though it is the violins who have the melodies and show-offy stuff, the violas have more fun!
