I feel the K-metering system sounds too low.  They say to master at K-12... but I swear almost every current song I throw into a a K-meter is like 2 to 3 db over K-12.  It seems like current music is being mastered at K-10 or something.

I agree about a K12 master not being loud enough also for some clients.  I work with other K levels which I have invented for mastering applications.
 
K-10 is a nice loud master and can still be punchy. It is just that there is only 10 dB of headroom for transients so they can start to have a not so snappy quality. It is where I master to one level up from K-12.  I have taken things to K-8 as well for some of the hip hop guys I have mastered for. Unfortunately they like it a little loud.
 
I am not going to make a K-8 master again. It might be loud but you are now really starting to destroy important qualities of the music and transient quality is one of them.
 
Once you try and attain levels higher then K-10 then the quality of the limiter really starts to take hold as well. PSP Xenon is fantastic for this sort of work. You can even shape the sound of the transients by how you adjust it. But you need to spend $250 on a limiter and be prepared to drive it pretty hard.
 
There are some standards in terms of LUFS being more ideal around the -16 LUFS. This is good for iTunes etc and many other delivery mediums. A consistent K System K-14 master produces very close to -16 LUFS so it is literally perfect for that.
 
A good CD to listen to is Steely Dan's 'Everything Must Go'  It is mastered around -11 to -10 and it sounds great. The perfect balance of loudness rms wise plus transient snap on things. It has a DR reading of 10 as well.  I still think a K-14 master has a great balance of volume rms loudness and transient quality.