Why don't we talk about ballad playing?
Ballad playing seems to be one of the most mysteriously under-discussed topics amongst drummers.  If you are going to be a professional musician in nearly any genre of music, you are going to have to learn how to play a ballad whether you want to or not.  So simply avoiding the topic is not an option. 

Perhaps the reason this topic is avoided is because the role of the drums in a ballad tends to be particularly understated/supportive.  You are not going to be the focus of attention when you are playing a ballad.  However, that in no way means that what you are playing (or not playing) isn't important.  On the contrary for a group to really pull off a ballad, what the drummer plays is key! 

Thoughts on how to approach a ballad

1.  Attitude
Don't be this guy
The first thing that matters when you are playing a ballad is your attitude.  You should never approach a piece of music as a chore.  Playing music as if you are being forced to is selfish and immature.  Here is a relevant quote from Art Blakey, "Music washes away the dust of every day life."  Music isn't just about you and your feelings, it is also always about your audience.

For me this is not an issue, because I really love playing ballads.  I am much to sentimental of a person not to get swept up in a beautiful ballad like "Sophisticated Lady" in the clip above.  I see playing a ballad as a unique opportunity to play the drums romantically.  You may be this kind of person as well, but regardless of your personal feelings, always remember to serve the music and the audience you are playing for. 

Do be this guy
2.  Play supportively
If you start with the attitude I described above, approaching a ballad as an opportunity to play romantically, then this step will follow naturally.  You should always strive to fit your playing to the mood of the song, so play as quietly, slowly, and in as legato a fashion as possible.  Don't try to fill every space, and only come up in volume or density of notes when the music calls for it.

This doesn't mean you that you should just play quietly and sparsely no matter what happens.  If you feel like the music is increasing in intensity, go for it!  Remember that loudness doesn't equal intensity.  In other words, you can play quietly and with real passion.  Listen to Chris Grasso's elegant piano intro, or Kriss Funn's beautiful bass solo starting around 2:03 for some examples of quiet but passionate playing. 

3.  Some specific ballad techniques
Nothing is more personal or subjective than brush technique, and this is particularly true in a ballad context.  That being said, here are some general pointers:

-You want a full sound that still accents the rhythm, so try to use the whole snare drum.  I use an outward motion almost like swimming breast stroke to accomplish this.  I will try to a video on this in the near future.

-Your bass drum and hi-hat need to come way down in a ballad.  Even though I normally play the hi-hat with my heel up to get a nice aggressive, "Blakey-esque" chick sound, on a ballad I almost always play with my heel down. 

-Speaking of the hi-hat, you should try to incorporate comping with the hi-hat and using the hi-hat splash.  The hi-hat splash is a great sound in general, but because of it's legato texture it works particularly well in a ballad.  You can hear me using these throughout the video. 

-Some soloists like to go to double time in their solos, some do not.  In the video, Kriss stays with the original feel, whereas Chris moves to double time.  I tend not to assume that a soloist wants to go to double time until I actually hear them playing some double time ideas.  Some drummers  launch into double time as soon as the solos start, but I have found that this can get you into trouble more often than it is worth.  This can be a delicate dance, so just rely on your ears and you will be ok.  You can even hear how we go briefly to a latin feel during Chris's solo at 4:43!  

-Endings on ballads tend to go to rubato, so listen carefully in the last A section.  The person playing the melody will generally cue the ending by slowing down.  In general you want to stop playing and give the melody time to wind down before playing a nice final swell, preferably with mallets.  You can watch this type of ending happen in the video for more clarity.

-Try incorporating brush "rolls" where you sweep the brush rapidly from side to side.  You can hear me play a brush roll going into the bass solo at 1:59.  This is a useful technique for a ballad because it can make a dramatic statement without being too loud.  Check out Jeff Hamilton doing this in his brush solo at 2:28:

4.  Get some ballad vocabulary
This should go without saying, but if you haven't checked some great masters of ballad playing, now is the time.  Here are some recommendations to get you started:

-Bill Charlap trio with Kenny Washington