I want to be able to write catchy/"cool sounding" guitar parts for songs. I am currently studying music theory extensively. I am just wondering, are there any guitar-specific musical concepts that I should look into to be able to write good songs, or is it mostly just feeling? I've been told that formulating a song with an emphasis on sound musical theory can make it sound sort of "manufactured". Any answers would be greatly appreciated.
Why not choose both? Knowledge AND feeling. I'd expect increased knowledge of music theory to give you more ideas to try, expanding the range of possibilities available to you. Use your feeling for the music to help you choose between them.
For creating 'catchy' parts on guitar look into:
The DESH (a major chord with a descending bass line) which The Beatles used to catchy effect. http://www.washedashore.com/music/desh.html
Counterpoint (melody parts moving independently) http://en.wikipedia.org/wiki/Counterpoint
I'd also suggest you study a bit of harmony and figure out all the different types of chords there are for you to use, otherwise you could be missing out on some emotive options. Then figure out all the different ways they can be voiced on the guitar (with and without open strings). It might take a while but I can guarantee you'll discover some 'cool sounding' options along the way.
And when you've learnt lots of theory don't forget that rules are made to be broken!
The central concept for guitar accompaniment is the "riff". This is the "atom" from which you build up cadences like molecules. The cadences will often correspond one-to-one with verses or phraes of the song.
The riffs themselves are little "nuggets" of rhythm and harmony that provide the backbone of the song (by defining the framework of the musical space). By analyzing your favorite riffs from your favorite songs, you can discover what "makes them tick" and then build completely different riffs using the newly discovered principles. Many of the best songs came about from failed attempts to learn other people's songs because this analysis/construct-new-riff phenomenon can often happen by itself, in the fingers, without conscious control! Being human is spooky.
As a guitarist, I love both my electrics and my acoustics. I find they have different, but overlapping, sonic spaces.
The main difference I would note between acoustic and electric, besides the obvious differences in tone, is that the acoustic is much more of a percussion instrument. Yes it has strings and a neck, but it produces its sound through this big wooden box, which will amplify anything else you do to or around it. So, rythmic tapping or slapping on the box, even as part of strumming, makes the acoustic a much more rhythmically expressive solo instrument than an electric, which although the vibrations and tonal characteristics of the entire instrument are important, is much more focused on the sound produced by the pickup transducers responding the the strings' vibration.
For evidence, take a look at what Michael Hedges, Kaki King, and Erik Mongrain do to their instruments when playing a song. Slap it, tap it, drum your fingers across it, it reacts to these inputs almost as much as to the strings. Even when just strumming, the rhythmic quality of pick hitting string and arm brushing past is reflected in the sound of an acoustic to a far greater degree than an electric.
One other thing is that your average acoustic guitar has strings of a heavier gauge than an electric. This is because the changing tension of the string as it vibrates is important to an acoustic; you can think of the top of an acoustic as a speaker cone which is being "driven" by the strings. A light-gauge string will not properly "drive" the top. However, heavier strings mean more pressure necessary to make the notes speak clearly, which generally means slower digitation; it takes better technique to be fast on an acoustic. Most acoustic players, compared to electric players, don't bother; they use more chords and create movement through rhythm and through simpler manipulations of the strings, which makes songs a bit more repetitive but also much richer tonally.
Lastly, I've also found acoustics to be much more forgiving of dissonance than electrics; the overall effect of close or uncommon intervals is more pleasing. Make no mistake, you have to tune it, but chords involving major and even minor seconds, tritones, etc sound more organic as all the various overtones of the strings' vibrational interactions are reproduced more fully than with an electric. So, "jazz chords" and other non-traditional voicings may find a better place on acoustic than electric.