I know that the label Nintendo Selects / Player’s Choice is applied to games that have sold more than one million copies and are sold for less, but do they ever have content differences such as more or fewer levels or characters, different graphics/sprites, bug-fixes, etc.?
As far as content is concerned, no. You're getting the same game that the previous buyers got, it's just a different box.
While there may sometimes be bugfixes, there does not have to be. Any bugfixes introduced in the Nintendo Selects version are necessarily minor, and would not affect normal gameplay. This is necessarily the case because there is no patching mechanism for the already released copies; if there are major bugs which severly impact the gameplay (such as the freezing bug in PAL copies of Super Paper Mario), the only way to fix them is by replacing the disc or cartridge.
Additionally, any bug fixes are not made specifically for the Nintendo Selects version, and may have already been released in previous pressings. Whether or not the two coincide is a question of when they switched the master copy; that may happen at the same time as pressing additional copies for the Nintendo Selects version, or it may not.
TL;DR: There are no notable differences.