Development Hell: Activision claimed that a new game in this series would be released in 2012. Assuming they were telling the truth, and the fact that no Guitar Hero game was released in 2012, it probably fell into this, and ended up being cancelled anyhow.
The now-cancelled Guitar Hero 7 would have featured dynamic venues, unique music videos for each song, and a new six-button guitar controller with strings instead of a strum bar, while removing drums and vocals. Your mileage will definitely vary on whether this would have been a good direction.
Warriors of Rock was originally going to include, among other things, the song "Deliverance" by Opeth. They decided against it due to disc space issues.
The first three games mostly used covers as opposed to the original songs. Awesome, sometimes hard to distinguish from the original, but covers nonetheless. Outside bonus songs, every song in the original game is a cover. Outside "Stop" and "John the Fisherman" (plus "Dead!" and "Possum Kingdom", plus some DLC in the 360 version), every song but bonus songs in Guitar Hero II is a cover. Same for Rocks the 80s, where the originals were "I Ran (So Far Away)", "The Hellion/Electric Eye" (okay, they credit only "Electric Eye" in the game, but...), "I Wanna Rock and "The Warrior" (and that's without mentioning "Because It's Midnite" which, for what it's worth, was as close to a bonus song as this game got). The widespread use of masters didn't start until GH 3 and the first Rock Band.
Ironically, the cover of What I Like About You was apparently so good, The Romantics tried to sue Harmonix over it. Which is doubly ironic when you consider it's not even one of the games' best covers.
As for the originals present in Rocks the 80s, since both "I Wanna Rock" and "The Warrior" end in fade-outs, the in-game versions also end in fade-outs. Considering that later games by Harmonix (read the Rock Band series) and later games in the GH series that use masters bothered to raise the volume of the fade-out and create an ending (yes, those endings are made by the developers - compare "You Give Love a Bad Name" in Lego RB and GH5), it's safe to assume They Just Didn't Care. Then again, this was their last game before Activision hijacked their own series from them, and while not really bad, the final product did seem rushed for a summer release, so that's possibly the reason not much work was put in here.
Of all the on-disc bonus songs in all the GH games, there's the sum total of one cover; "She Bangs The Drums" in GH3.
The Sex Pistols actually reformed and recorded new versions of two of their songs ("Anarchy in the UK" for GH3 and "Pretty Vacant" for World Tour) specifically for the game (the original masters were probably unusable in the game for technical reasons). Well, for this and for the same reason bands always reform.
Technical reasons like not existing. The theory the band had was that the masters were stolen.
Hyperspeed was originally included as a joke, like the giant heads "cheat". Yeah, let's make the charts scroll twice as fast! Then people started using it, and noticing the faster speed makes them much better at the game than without.
Guitar Hero Smash Hits (Beenox) and Guitar Hero Van Halen (Underground Development) were handled by third parties, which would explain some of the chart discrepancies between those games and earlier appearances. In the case of Smash Hits, it's mostly because masters were used instead of covers, but for Guitar Hero Van Halen (which has Hot For Teacher, which appeared on World Tour but didn't export), there really isn't much of a reason to change the chart itself if you're using the same masters.
...and on the other hand, some of the charts were changed just to make the tracks more assholish. The five-note chord at the end of "Raining Blood", anyone?
For Smash Hits, it's worth bearing in mind that some note types were added in World Tour which weren't in previous games (tapped notes, extended sustains, open notes on bass). This meant that in songs which weren't covers in their initial appearance (such as Miss Murder), various parts could be represented more accurately. This also applies to One in Guitar Hero Metallica.