Actually, on software-decoding targets, supported codecs are identical, the vast majority of plugins are supported on all of them (and those that aren't, should be soon), and the keymapping (as well as included plugins, again) is covered in the manual (where users should always be encouraged to look) so player-specific help should *always* be preceded with "What you're trying to do is covered in the manual. Please, try to follow its directions. If those don't help, come back and tell me exactly what you pressed to try to get it to work, and what player you're trying this on."
The whole idea of the forum redesign was to get people away from thinking about Rockbox as "Different Players Running Rockbox" but rather as a software that has different hardwares under it. The primary difference between any given build are whether they have recording, FM, a clock, or a color screen, and their button maps.