Stylistic Differences in Glides and Drops
Different styles of music can have different styles of glides. Most of the effects below occur on the long notes; it is difficult to produce them on the short notes.
The "generic" glide is an interval of about a third, occuring fairly quickly at the start of the note. This is found in "pop" music, such as rock and Broadway musicals. Listen to the Beatles, who almost define this style.
As far as I know, the only distinctive feature of crooning is the a large glide. If the typical glide is around an interval of athird, the typical croon is about a fifth. For example, I would start "White Christmas" in a Bing Crosby style by gliding up from the fifth below to the third (G to E in the key of C). I would start "Thanks for the Memories" in the Bob Hope style with an octave glide.
Perhaps these glides need to be longer in duration simply because they are larger in size. But I think the crooners also increase the duration of the glide, to further emphasize it.
The singer in "Simple Plan" (a debateably punk band) tends to have a normal size of glide, about a third, but to prolong it to the point of obnoxiousness. As noted above, I called it "suspenseful" when it takes a long time to resolve the glide to the correct note. Another word for this is "annoying". The lead singer almost gets to the correct note, then holds this off-chord note. The note he is singing is not harmonic, it is annoying. He then resolves the note to the correct pitch, but the overall effect is annoying, kind of like whining. (And I really like their famous CD, No Pads, No Helmets, No Balls.)
The characteristic glide in Country music does not make it to the correct note. Instead, the note stops just before reaching the correct pitch. Now that creates a mystery -- how can something be the correct pitch of a note if it is never song? As far as I know, the correct pitch is implied, so people somehow know what it is.
Why isn't the Country glide aggravating like the Punk glide? I think it is because there is no holding of the final dissonant note. Instead, it is like a normal glide, except the singer just stops singing before actually getting to the proper note.
I suspect that many country singers in fact will make it to the proper pitch. However, as I hear it, the most country-sounding glide does not. (There is of course more to Country and Punk than just the glide.)
Blues characteristically has a pronounced drop at the end of the note.
Other Stylistic Variations
Crooning, Country, and Punk are particular styles of music with different exaggerations of properties of the glide. Other styles of the glides and drops are within the "normal", so these tend to be classified as pop/rock.
Glides on Every Note. Glides usually occur on longer notes and starting notes. But they can also be put on every note.
As near as I can tell, Elton John does this. For example, his first big hit begins with the line "It's a little bit funny" There seems to be a very noticeable glide on every note, even the second syllable of funny. Perhaps parenthetically, I found that particular song completely unsingable until I discovered glides. (Actually, I don't know how Elton John actually sings this song, I just know that when I try to sing it I need to use his style, and that style seems to be gliding on every note.)
Drops. Prototypical rock/pop has glides but not drops. Some modern singers, especially female soloists, put more emphasis on drops.
I don't even know if Avril Lavigne does glides, but she makes good use of drops. Typically, the last verse of her songs doesn't have a drop on the notes, which makes it sound much stronger (in contrast). She also sings some of her drops, which I find heavenly.
Kelly Clarkson has glides but the drops seem more important. He drops are a little stronger than Karen Carpenter's, so there is a little bit of singing added to the drop.
Caressing. If there is a glide and a drop on the note, then the note can sound caressed. Jim Croce for example does this.
Warbling. To exaggerate the harmonic note of the drop, you can hold it at a steady pitch, instead of just gliding from it. You can further emphasize it by going straight from the harmonic to the correct pitch, and you can even further emphasize it by putting a break in sound in between the harmonic and the correct pitch.
If you do all of these, it sounds like what I call warbling. If you just hold the harmonic, you probably won't have much time for a glide, and it will still have a warbling effect, though not as strong as if there was a sound break.
For example, Gordon Lightfoot has some complete warbles, but most of the time he is doing more subtle warbles -- he holds the harmonic, but there is no sound break between the harmonic and the correct pitch.
DionThere are some singers, such as Lind ("Elusive Butterfly") and Dion ("Abraham, Martin, and John", probably "Runaround Sue") that I find difficult to analyze. There are doing something right, but I don't know exactly what. I think maybe possibly that one of the things Dion does is a very small glide into a note. Assuming harmony, this implies a more complex chord. For example, if you glide from the fourth into the fifth, you could be just singing the modern second-fourth-fifth chord.