Hmm. Like Peardian, I'll group these by interest level, since many could go either way:
-Unused Stages/Areas/Levels/Places/Locales/Synonyms
-Unused Playable Characters
These two are really impressive bits of cut content. Entire areas and playable characters having gotten the axe suggests some fairly significant changes in development... and really makes me wonder *why* these things were cut, especially if they work perfectly.
-Unused Enemies/Bosses
-Unused Items/Abilities
-Unused Music/Sounds
These are always fun. Oftentimes, the unused music turns out to be extremely good, leading one to wonder why it never got used... same with unused items and abilities, which often work perfectly, and would've been great had they been included.
-Debugging functions
Why so low on my list? Honestly, it depends. If the debug mode allows you to explore the game in new and interesting ways, such as the ones in Link's Awakening or Lufia 2, I'm all over it. But a lot of debug modes are just stuff like "invincibility" or "stage select", which while fun, can be accessed with PAR codes *anyhow*... or even things like "hardware tests", which are totally useless to me.
-Unused Graphics
-Unused Text
Just curiosities, mainly. Unused story text can be fun, giving you insights on plot elements that got cut for whatever reason. I admit, I like ripping unused graphics and using them in my RM2K games, but as an actual unused element in a game... they're not exactly useful.
-Regional Differences
-Revisional Differences
It's always interesting to see what was changed between versions... though I feel like we're getting a little nitpicky with these of late, documenting everything from title screen changes (of course those are going to be different), to every minor text change, to extremely slight changes in color. Big differences, such as items that were completely different, drastically changed graphics, or entirely redesigned areas are fun and interesting. "PASS WORD" -> "PASSWORD", much, much less so.
-Anti-Piracy Measures (As in stuff built into the game to hinder you)
These could be interesting, if not for the fact that almost all of them are actually disabled by default with no way other than cheat codes to re-activate them... The Earthbound stuff is quite impressive, however.
-Unused Development Text (Hidden developer messages, build dates, code, etc.)
...bleh. While hilariously angry developer rants such as the one in Pachi-Com are awesome beyond awesome, almost all developer-related text is very mundane. Build dates and uncompiled code mean little to me (I don't even know what the code is supposed to *do*), and ... eh. Like I said on IRC, I won't lobby against these things, because they *are* unused content... I just don't *like* them, personally.
-Hidden Sound Tests
With all the ripped music formats (and MP3s) out there, these just aren't so useful. Though admittedly, I've been able to use them to grab SPCs that were missing from the sets in SNESMusic.org... so I can't complain *too* much. 
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