A most excellent guide Doctor Gravity. However, playing devil's advocate, I've not found knockback to be needed with my grav/ff con. The philosophy I took with him was, "I do not do damage. I hold. I bubble. I do not solo. I am loved by my group. I rock."
Since I don't damage, lift & propel aren't worth the power slots. Since I am loved by my group (the scrappers & blasters in particular) knocking back or making the mobs intangible isn't very efficient. Since I bubble, my scrapper allies become mini-tanks (the leadership maneuvers & tactics help here), and they don't need the enemies pushed away (though I do stay closer to the melee than one might deem safe). Since I hold, I take pleasure in watching teammates practice their pinata skills. I rock.
I have just hit 32 and I can now add, "I love my pet singularity." It makes my holds that much more efficient for lts & bosses, since they have to be held twice successfully. The pet still hardly does damage, and I'm fine with that. I'll grant you without question that the singularity bomb-knockdown when casting it in the middle of a mob... oh so fun and rewarding.
So yeah, I'm not knocking (pun!) the zen of knockdown, I'm just saying the grav/ff combination makes the melee safe enough where you don't need to push anyone away. That said, I still have force bolt, for those utility times when I need to take a guy off of my blasters.
Just to reitterate, Doc Grav, you rock.
