I know that a contract does not need to be signed. In general what counts as accepting a contract? I know some people can be tricky with words. For example if you emailed someone a contract (for example agreement to do freelance writing) and they reply "this seems sufficient" does that count? Or are the words "this seems sufficient" too ambiguous to accept a contract?
What counts as an offer being accepted?
Any conduct that reflects a reasonable conclusion that the offeree knowingly and willfully approved of the terms and conditions of the offer.
Acceptance can be manifested verbally or in writing (as the other answer rightly pointed out); alternatively, it can be inferred from the offeree's subsequent conduct that indicates (1) his enjoyment of benefits that are attributable to the contract, and/or (2) his efforts to comply with his duties as per that contract.
The language "this seems sufficient" constitutes an expression of knowing and willful acceptance, and it will ordinarily be construed that way unless the offeree is able to prove/persuade that the expression referred to something other than actually entering the contract.
This and many other aspects of contract law are similar among many jurisdictions: all of the US, Canada, many European countries, and elsewhere. The Restatement (Second) of Contracts thoroughly formulates the doctrine of contract law, and it is frequently used as reference in opinions released by U.S. courts.
For a contract to be valid there needs to be an offer and an acceptance. This can be verbal or in writing. In this case, emailing a contract would be an obvious offer and the reply "this seems sufficient" to me would be understood as an acceptance. The reason that I would consider this to be an acceptance is because it is generally understood that once a contract is proposed the opposite party will either accept the deal, reject it and propose a new contract, or reject it entirely. Also it's worth noting that different states have slightly different laws on contracts.
I'm not a lawyer, this is just my take from some light reading on contract law.