My favourites:
*Mad preacher on Omega (You sir, you are a blight! and you, and you... and YOU)
*The turian in citadel security (You sir are all racist!), and the dumb human that doesn't let him in, thinks an asairi is a geth, and then lets a geth whose class is "geth infiltrator" in instead
*I am a biotic God!!!
*The hamlet and bouncer elcor
*Blasto (he, or some mecenary hanars, or actually any talking hanars need to appear in ME3 or DLC (Kahje?

)
*The turian and quarian on illium
*Punching the reporter and the refound guy (where is the shifting looking cow BTW?)
*The action adventures turian on the citadel (just don't kill the council)
*The asairi on illium
*The krogan woman showing interest in Shepard
*All of Joker and Mordin
*The conversation between Garrus and Tali (Do you remember all those conversations we used to have on the elevator)
That's about it.

Bioware has to consider at least expanding on all of these in expansions and ME3.