I've been taking arranging and composing classes at the college I'm attending and I have recently been tasked with writing a piece for a string quartet. I would like to know in general, what should someone be aware of while composing/arranging a piece for a string quartet?
MattPutnam's answer covered technical aspects very nicely. Here I have some further thoughts that are often overlooked.
String quartets require you to be careful about more than notes. Even though the three (there are two violins in the quartet) instruments belong to the same family, they have each their own perks: they respond differently to dynamics; their articulation and enveloping (attack, decay, sustain, release) can vary a lot because of string gauges, region of the neck being pressed, bow size vs. pressure etc.
You can achieve various effects using the same notes. The middle C will sound warm but a little bit scratchy on the violin. The viola will get a velvet-ish tone, very smooth and nice, responding well to virtually every bow pressure. In the cello, you get a poignant, brittle, strained sound, carrying a very different emotional status.
That said, we get into two very important considerations:
The viola and the cello are there not just to "fill" into your voice leading
You can (and should) call them into action every time you want to make a given note stand out and the violins are busy or cannot sound very clear (on a low register, for instance). The viola is often overlooked as a "long note, lower voice" instrument, but its higher register can be used to great dramatic effect, while letting room for the violins to get "dreamy" into their ethereal treblest notes (think of harmonics, sul ponticello or low bow pressure [AKA flautando]). The same way of thinking is valid when you want some melody to "emerge" from the lower voices. The cello going up and getting to its highest register (from G2 up) gets even greater drama and aggregates more "weight" to any melody intended.
"First violin" and "second violin" are not "solo violin" and "base violin"
You really should alternate the violins into the leading role (includes overlapping them, making them "conversate" contrapuntally etc.), for several reasons: first, there are two different violins, with some timbre variation which can enrich your arrangement. Second, there are two players, who can desire either some prominence in your arrangement and also some rest from rapid or intense solo passages. Making your violins cooperate is nice to the overall feel of your music, as it will preserve timbral richness and players' stamina.
Other thoughts relate to the ways of playing. Aside from ordinary (arco) playing, you can resort to a wealth of different sounds. To name a few: pizzicato, sul ponticello, sul tasto, con sordina, col legno [battuto], percussion on the resonance box, playing behind the bridge etc. If you are not familiar with these names, I suggest you to do some research (either google it or read an instrumentation/orchestration manual as Adler's) and get what they sound like.
These are not rules of thumb or guidelines, but the result of years of learning (reading and listening to the masters) that can turn into practical advice. I think the best way to get all from your quartet is by not overlooking any possibility and giving clear assessment of the role of each instrument. From this point, it is all bound to your creativity and skills.
Aside from all of the standard voice leading stuff, all you really need to be aware of is the physical limitations of the instruments. And for strings, there's not a lot that you have to avoid. Know the tunings of the strings, so you can avoid writing impossible double stops (two notes that must be played on the lowest string). Commonly, people write boring string quartets because they're scared of writing something too difficult, but you honestly have to try to write something unreasonable (aside from horrible double stops). Strings are remarkably agile.