Plenty of better thinkers than I have tackled the subject of open theism and shown its demerits at a purely Scriptural and theological level. I'd like to take a few words to demonstrate its logical inconsistencies. I hope you'll pardon the use of a slightly more formal logic than I would ordinarily use in a post.
An important definition before we begin:
Libertarian free will - the view of free will held by open theists, positing that man's will is entirely continguent and therefore unknowable. That is, it cannot be known, even by God, until the choice is made.
Axiom: Scripture is true.
Corollary: God knows that some will be saved (see Revelation, etc. for demonstration of His sure knowledge of the salvation of a bride for Christ).
These two may be considered "fundamental" in the sense that they are agreed upon by both open theists and traditional Christians. The first is truly axiomatic; the second we take as equally foundational since it logically follows from the first.
Axiom: Scripture allows the view that man has libertarian free will.
Axiom: Scripture does not require that man has libertarian free will.
Here is where the point of contention is, of course, but most open theists agree that libertarian free will is not required by Scripture. Most traditional Christians would contest the first of these premises, but we posit for the sake of argument.
Premise: Man has libertarian free will.
From the premises we argue:
1. God, not knowing the future, does not know what individual men and women will choose regarding salvation until they have chosen.
2. All men, their wills being free, may choose to go to hell or to heaven by accepting Christ's salvation and Lordship. (Clarification: this is all individuals.)
3. The general case of some persons going to heaven is contingent on the choices of individuals to go to heaven.
Therefore, since all individuals may choose to go to hell, and their choice may not be known beforehand
4. God may not have knowledge of some people's going to heaven absent knowledge of specific individuals' going to heaven.
5. God cannot know whether any people will go to heaven, and therefore whether Christ will have a bride, people of every tribe and tongue will be saved, and so on.
Contradiction to an axiom.
Since the axiom is not under debate, nor its necessary corollary, it falls to us to reject our premise as false. The axioms may come under consideration separately, but assuming those, we must reject the premise that man has libertarian free will.
Why is this important? Because libertarian freewill is the keystone of open theism. Open theism posits that God's lack of foreknowledge of the future is because of the indeterminacy of the future - that is, that it does not yet exist, because it is contingent on the choices of man. Specifically, the argument is that because of libertarian free will, the future may not be known. However, if man does not have libertarian free will, then the future may be known (or else another reason why it may not be known must be shown), and indeed is known by God, who (all agree) possesses all knowledge that may be known.