From a technical standpoint, which is what the OP asked about, some things that come to mind are:
1. How hard you hit the strings
Here we're talking about tonal changes, not volume.
A softer touch "feels" gentler and quieter, a harder hit maybe with a little fret buzz sounds more intense and aggressive. Usually you want to hit different beats in a measure with slightly different intensity so it doesn't sound mechanical.
2. Dynamics
Here we're talking about volume, not tonal changes.
Generally you want to play quieter during the verse and louder during the chorus. Raising the volume generally raises the intensity.
3. Sustain
Longer sustain gives a more modern feel, shorter thumpier notes (especially on a P Bass with flats
sounds more oldschool.
4. Effects
Heavy compression can smooth things out so much it sounds almost tranquilizing. Overdrive wakes people up. Phasers, Flangers and Choruses promote a kind of trippy, psychedelic feeling.
5. Fingers vs. pick vs. slap
Each has its own feel.
6. Meter
Playing behind the beat, i.e. very slightly after where a metronome would put it, gives a laid-back, lazy feeling. Playing ahead of the beat can sound frantic, like punk rock. This does not change the tempo of the song, just how much you "miss" hitting the note exactly when a metronome would.
7. Rubato
Varying the tempo of certain passages in a song, especially a solo.
8. Register
This is why some folks downtune. Low notes feel more powerful. To me low notes are best used to reinforce the feeling that the guitars and vocals are putting out. Playing way high up on the neck will bring you out front, which is appropriate in some parts of some songs.
9. Left-hand looseness
When you get to where you can play a song with your left hand very relaxed, you can introduce subtleties that convey emotion. Notice that the note sounds different if you fret it right up snug by the fret versus halfway back. You can shake your left hand back and forth to introduce a little vibrato (watch violinists and copy them).
10. Brain
A song usually conveys a single thought, a single emotion. Close your eyes and feel that emotion like a method actor. Think of a time in your life when you had the feeling that the song is trying to portray. Exaggerate it in your mind, close your eyes, and play.
Exercise: The violin motif from the movie "Young Frankenstein". Experiment with Rubato, Vibrato and Dynamics. Imagine the feeling of the professor longing for his monster to come home.
Exercise: Born to be Wild by Steppenwolf. During the intro, hit the strings a little harder in certain parts of the measure to emphasize them. Try to keep the volume even. Dynamics - softer verse and louder bridge & chorus.
Experiment with overdrive or fuzz
Exercise: Will There Be Enough Water by The Dead Weather. The song lays back behind the beat and uses low register on the piano to create a dark, yet sleepy mood.
Exercise: Next to You by The Police. Play ahead of the beat to create a rushed, frantic feel.
