A good note topic will 1) make a claim, that is 2) novel, 3) nonobvious, 4) useful, and 5) sound to both the writer and the potential readers.* Novelty is essential and is addressed on the 'Avoiding Preemption' tab, while obviousness, usefulness and soundness are judgment calls that will depend on the topic and on the skill of the writer.
What we will focus on here is the development of the claim - the argument that the writer of a law review note intends to make. No matter what the particular argument is, claims should be straightforward and short - you should be able to set out your argument in a single thesis statement.
The first step to coming up with a claim is to identify a problem - conflicts in the law, gaps or errors in current scholarly understanding, or any other flaw or hole that you identify within the legal world as it stands. The next step is to propose a solution to that problem - your proposed solution then becomes your claim. The claim may be as simple as identifying an unconstitutional law ("Law X is unconstitutional because of Y") or more complex ("This law X is flawed when viewed from Y perspective, and should therefore be amended is this way to address concerns A, B and C") but should in all cases be simple enough to describe simply. This approach helps prevent topics that will be poorly defined or too broad to handle down the road.
* framework and examples adapted from Eugene Volokh, Academic Legal Writing: Law Review Articles, Student Notes,Seminar Papers, and Getting on Law Review (3d ed. 2007).