I'm actually thinking it's some form of momentum vector and determination of whether the atoms/molecules are purposely trying to go through the event horizon, which is cool.

Yes, some kind of cohesion combined with *purposeful* movement, rather than random motion.

Aren't atoms held together by electrical attraction? Okay, not electricity, per se, but... (flails for the right word). Same is true of molecules, which are attracted one another by filling in the positions ions (ions? eh, maybe) leave open (kind of like arms)

So, I'm thinking maybe the gate picks up on that basic electrical cohesion of matter, distinguishing it from random... stuff.

Which also means that if something came up with the right weapon, they could disintegrate us like replicators, by counteracting the electrical bonds holding our molecules together. Which is maybe what that weapon on Dakara did.

Also, re: the Stargate 'sucking'. Remember the effect from the movie? Where people stick part of themselves in the Event Horizon and the stargate sort of 'sucks' them in from there? Yeah, they're duplicating that effect in the Ark of the Truth. Which... isn't really a spoiler of any significance, I don't think.

So much for the Stargate not sucking ;)