What makes something a take-it or leave-it contract?
The lack --be it essential or literal lack-- of opportunity to negotiate the terms of a contract. That is also known as adhesion contract.
And (if I am the one taking it), are unclear clauses categorically interpreted in my favor?
Rather than "categorically [interpreted]", a more accurate characterization is to say "reasonably [interpreted]". Among the reasonable interpretations of a contract, the adopted one is that which favors the party who was not the draftsman of the contract. This is known as the doctrine of contra proferentem and is cognizable in jurisdictions of the U.S. and in many others world-wide.
The doctrine of contra proferentem is sought to compensate for the gap of parties' bargaining power. In an adhesion contract, the fact that one party may only accept the contract "as is" (or reject, which would render this a moot issue) evidences that the draftsman of that contract has significantly more bargaining power.
A contract need not be one of adhesion in order to trigger the doctrine of contra proferentem, though. In the case of negotiated contracts, the doctrine may apply selectively so as to interpret ambiguous clauses against the party who ultimately caused the ambiguity in those clauses. Some jurisdictions are more emphatic about this aspect of contract law, which at first glance may seem a departure from the principle of interpretation [literally] against the draftsman.
There could be scenarios where a party (the non-draftsman) suggests a clause, and the draftsman incorporates that clause but devises some wording to render the contract unclear in that regard. The doctrine would apply not merely by virtue of the latter party being the draftsman, but because he in his capacity of draftsman tweaked the clause arguably in an attempt to frustrate the other party's interest in --or purpose for-- that clause.