When reading the intial posting, I suspected that the "open chords" in question were first-position standard-tuning chords rather than open-tuning chords. As I continued to read, and saw all the contributions from open-tuning enthusiasts, I began to think I might have been mistaken, but eventually learned that we are being asked about those basic standard first-position chords.

So here goes:

As you continue to learn by playing and listening, you will come to recognize the different sounds of riffs characteristic of the various basic keys, as played in standard tuning with open chords. Just a few examples:

In D, when playing the tonic D chord, it's hard to avoid the temptation to fiddle with that top E string ~ fretting at 2d fret as part of the standard chord, then open, then fretted at #3 with your pinky. Rinse/lather/repeat. It's a unique little sound that can't be duplicated in any other key ~ except of course with a capo. (Not easily, anyway.)

In E, on the E chord, you can get a nice blues effect by hammering-on and pulling-off your index finger on the G string (technically, switching quickly back and forth between Emajor and Eminor).

The keys of C and (to a slightly lesser extent) G are the best ones for easily "walking" the bass line, using your middle and ring fingers to fret the three bass strings at frets # 2 and #3 (and open). Playing the same bass lines in E, A, and D is tougher because you have to reach the 4th fret with your pinky ~ damn near impossible while keeping your other three fingers held down tight on the basic chord. C and G also offer relatively easy opportunities to pick melodies on the top two (sometimes three) strings, open and at frets 1 and 3.

Incidentally, you can reach more notes more easily if you play the G major chord with middle, ring, and pinky fingers than if you use the awkward hand-turned-over G chord usually taught to beginners. Fret the bottom E string at the 3d fret (low G) with the ring finger, the next string (A) at the second fret with the middle finger, and get the high G (top E string, 3d fret) with the pinky. This makes it way easier to switch back and forth between G and C chords, to do the abovementioned "walking bass," and to play melody/treble notes on the top strings at the first and third frets (with index and pinky) and on the open strings.

I feel that I became a much better guitar player instantly on the day that I learned this alternative fingering for the basic open G chord. I've never used that other fingering since, and don't think I could play a G chord that way now if my life depended on it!

Another little trick I picked up at about the same time as I learned the new G-chord fingering: play the D (or D7) chord by sliding the C (or C7) chord two frets up the neck. If you're not playing the 7th, it's generally best to fret the high E string with the pinky (at 5th fret), because the note E has no business being part of a D chord. (As far as the low E string is concerned: skip it, mute it, or fret it at the 5th fret with your ring finger if you can ~ some players with big wide fingers can hold down two bass strings with that finger; I just alternate playing the two bottom strings, one at a time, fretting at #5 with my ring finger.) The open G string is not truly part of a D chord either, but it sounds OK as a "drone" open note when playing in the key of G, not unlike the fifth string on a banjo.

This same chord form (slid-up "C" as D) also works as an alternate Eminor chord if both top and bottom E strings are sounded while the "D" string is muted (rather than fretted at #4). This chord occurs in lots of early Bob Dylan (e.g., "Girl From the North Country" and "Boots of Spanish Leather") and, I assume, in the traditional material that Dylan was borrowing from at the time.