The four tests, and the three things a note may add without failing them. UCC 3-104(a) defines a negotiable instrument as "an unconditional promise or order to pay a fixed amount of money, with or without interest or other charges described in the promise or order", if it "(1) is payable to bearer or to order at the time it is issued or first comes into possession of a holder; (2) is payable on demand or at a definite time; and (3) does not state any other undertaking or instruction by the person promising or ordering payment to do any act in addition to the payment of money". That third test would disqualify almost every real loan document if read strictly, so the same paragraph permits three additions: an undertaking or power "to give, maintain, or protect collateral to secure payment", an authorization to the holder "to confess judgment or realize on or dispose of collateral", and "a waiver of the benefit of any law intended for the advantage or protection of an obligor". That is precisely why a mortgage note can pledge a house and stay negotiable, while a note promising to pay money and deliver a car would not.
Non-negotiability can be an accident or a choice. A note tied to a contingency is conditional and fails the first test; a note payable "when the property sells" has no definite time and fails the second. Drafters also opt out on purpose: 3-104(d) provides that a promise "is not an instrument if, at the time it is issued or first comes into possession of a holder, it contains a conspicuous statement, however expressed, to the effect that the promise or order is not negotiable or is not an instrument governed by this Article." Nothing about that makes the note weaker between the original two parties. It changes what happens when the note moves.
What negotiability buys is a buyer who is hard to argue with. When a negotiable note is transferred to a holder in due course, UCC 3-305 splits the borrower's possible defenses in two. Paragraph (a)(1) lists the defenses that survive against anyone: infancy, "duress, lack of legal capacity, or illegality of the transaction which, under other law, nullifies the obligation of the obligor", "fraud that induced the obligor to sign the instrument with neither knowledge nor reasonable opportunity to learn of its character or its essential terms", and discharge in insolvency proceedings. Paragraph (a)(2) and (a)(3) cover ordinary contract defenses and claims arising out of the transaction itself, and (b) says a holder in due course is "not subject to" those. The commercial purpose is that a note can be sold at a price that does not depend on investigating the deal behind it. The consumer consequence is that a borrower's complaint about the seller can end up being no answer to the finance company holding the paper.
Consumer law takes most of that back, and it does so twice. The Federal Trade Commission's Holder Rule at 16 CFR 433.2 makes it an unfair or deceptive practice for a seller to take a consumer credit contract that omits a notice, set in at least ten-point bold face type, reading "ANY HOLDER OF THIS CONSUMER CREDIT CONTRACT IS SUBJECT TO ALL CLAIMS AND DEFENSES WHICH THE DEBTOR COULD ASSERT AGAINST THE SELLER OF GOODS OR SERVICES OBTAINED PURSUANT HERETO OR WITH THE PROCEEDS HEREOF", with recovery under the notice limited to amounts the debtor has paid. The rule is tied to a sale or lease of goods or services to consumers, and a parallel version at (b) covers purchase-money loans; it is not a rule about every consumer loan. The UCC then closes the gap left by a seller who simply leaves the notice out: under 3-305(e), where other law requires such a statement and the instrument does not carry it, "the instrument has the same effect as if the instrument included such a statement", and (f) subordinates the whole section to other law establishing a different rule for consumer transactions.
The same instrument keeps reappearing under different names. A home loan produces a note plus a mortgage or deed of trust. Federal student loans use a master promissory note, one document covering loans made over several years. A loan between relatives is documented by a note, which is also what separates a loan from a gift when the Internal Revenue Service looks at it. A seller-financed sale produces a note payable to the seller, secured by the property sold. An estate-planning installment sale to a grantor trust produces a note payable to the seller of the assets. A shareholder advance treated as a loan needs one for the same reason. In each case the surrounding rules differ completely and the instrument is recognizably the same object: a signed promise, for a stated sum, payable at a stated time.
One caution about the source of all this. The Uniform Commercial Code is a model act drafted by the Uniform Law Commission and the American Law Institute, not a federal statute. The section numbers and text quoted here are the uniform text; what governs any particular note is the version of Article 3 the relevant state has enacted, and state enactments differ in wording and in which later amendments they adopted. Two notes with identical terms can therefore produce different results in different states, and the state's own code is the text to read before relying on any of it.