The viola is truly one of the often overlooked joys of the world of string instruments. Like all instruments, it takes years, a lifetime to master. However, for the masses already well versed in violin technique, the viola is extremely accessible and one can, with work, often equal or even surpass their violin ability on the viola.
The bow of the viola is heavier, thicker, and harder to control than a violin bow. So long as the player already has a suitable bow grip, no changes need to be made in that respect. One of the first changes I made when learning to play the viola was rosin use; as a violinist, I barely ever used rosin, applying it at monthly intervals at the most frequent. Because I never used rosin regularly, I was able to adapt my technique to account for this "deficiency," and it never became a problem. However, appropriate rosin use is essential when playing the viola. Because the strings of a viola are thicker and tougher than those of a violin, a bow without enough rosin will create the thin quality of tone that many will expect of violinists who play the viola (!)
The other key element of bow technique for a violist is economy of bow usage: bowing with a full arm and using upwards of half the bow for runs of eighth notes, for example, may be more comfortable for some, and is acceptable on the violin. However, even if a proper tone could be attained by playing in such a way on the viola, one could scarcely play one page in such a way before becoming too tired to continue! As little bow as possible, with as few movements of the arm and shoulder, while still maintaining a good tone and adhering to stylistic standards, must be used when playing the viola.
Bowing on the viola is all about digging in. Don't be afraid; even a piano passage needs to be truly played. Get "into the string" with every stroke of the bow.
It cannot be said enough: the viola is heavy. I've had, at the age of 14, lower back pain from playing it for extended periods of time. Which leads into one big point about playing the viola: If you are one of those people who holds the instrument parallel (or higher) to the floor, I hope you're strong. I'm not saying that you should let your arm drag and play with the viola pointing at the floor. Still, it is important that you don't needlessly exert yourself.
It's important on the violin that your fingers are as "high" as possible on the fingerboard, that only the tip of the finger makes contact with the string, that your knuckle line is above the neck of the instrument, and that as you play on lower strings, you swing the elbow to the right to accomodate for the extra distance you're reaching across the neck. On the viola, these things are imperative. The neck of a viola is thicker and wider than the neck of a violin, so to avoid strain on the hand, your elbow must move with your hand, and your fingers must attack the string from above with the tip of the finger only.
In addition, the viola is longer than the violin. The combination of the added width and length make the notes on the lowest string, C, especially difficult. The only way to make play on the C string comfortable is to bring the thumb forward, towards the body of the instrument. Often, even while playing exclusively in first position, you'll need to move the thumb to reach low notes with the first finger or high fourth fingers.
Alto Clef - The majority of viola music is written in the alto clef, with the only departure being that as opposed to resorting to ledger lines as in violin music, the higher notes on the A string are usually written in treble clef. See the alto clef for information regarding learning the alto clef.
Repetoire - The first piece I learned for the viola was Telemann's concerto in G major. This is a wonderful introductory piece for the viola, as it is not only fun to play and quite beautiful, but also encompasses many different styles for the violist to become accustomed to. Sheet music for the concerto can be found in the Suzuki series book Volume 4, which also includes several transcriptions of violin pieces, most notably the Vivaldi concerto in A minor. After the Telemann concerto, his 12 Unaccompanied Fantasias for Viola are a great advanced collection of pieces that sound spectacular.
Etudes and Scales - The generally accepted viola scale book is Mogill, which is a transcription of the violin scale book Hrimaly. It includes major and minor scales, arpeggios, and finger patterns. Even if you already know your scales on the violin, becoming familiar with scales on the viola with Mogill will greatly improve your intonation.
As for etudes, I used book 2 of Wohlfahrt to improve my intonation, understanding of viola position work, and also to become more accustomed to reading the alto clef. Wohlfahrt was transcribed for viola by Merle J. Isaac and Ralph C. Lewis, and they do a wonderful service for those learning the alto clef through their transcriptions. In some cases, a high passage will be written out in alto clef with ledger lines, but if that passage is repeated in the same etude, they will then write it out in treble clef! It's extremely important for a violist to be comfortable switching between alto and treble clef as they play, and the transcription of Wohlfahrt is a perfect way to do this. Another advantage of using Wohlfahrt, if you used it on the violin also, is that you will be at least fairly familiar with the etudes. Thus, especially when you are starting out, you will have an idea of how each one should sound, and less guess work will be involved with playing.