Glides and Drops
Jim Croce tends to put glides on the start of his notes. This is a typical glide, from a harmonious pitch to the correct note. There is no exaggeration, such as found in crooning; instead, it follows typical rock/musical style, and if anything might be understated.
He also puts small drops on the ends of his note. Again, the drops are apparently to a note in the harmony. These drops are typical, and if anything small. I know of no instance when he sings the drop.
But, as far as I know, both gliding into a note and dropping it at the end is somewhat unusual. The result is that it sounds like he is "caressing" each note.
This also means that he is not spending a whole lot of time on the correct note. Combined with the fact that he apparently sings with a southern accent (even though he is from Philadelphia), his songs have a slight "country" feel. But, country music typically does not even make it to the correct note, and Croce never does that. (Additionally, his guitar accompanients tend to be complex and sophisticated.)
Jim Croce sings fast songs and slow songs. The fast songs include his famous "Bad Bad Leroy Brown"; the slow songs include "Operator" and "Time in a Bottle".
In general, for a fast song, Croce does an excellent job of being rhythmic. Which is to say, he starts the notes on the beat. As far as I can see, there is no use of the more difficult technique of using the closing consonants of a syllable for rhymic purposes. To the contrary, Croce usually minimizes (or even drops!) the final consonant of a syllable. This is consistent with his drop. A drop is produced by loss of vocal effort, so the end of a syllable, where the drop occurs, is likely to be soft.
I suspect that this de-emphasis of the final consonant increases the rhythmicity. In other words, the ultimate in rhythmicity would be to use the final consonant in the rhythm. But if you are not going to use it, but instead put the consonant in any random place, then you are better off minimizing or dropping that consonant. Then you have mostly just starting consonants, which in Croce's case, are very rhythmic.
The one exception to his "fast songs are rhythmic" rule is, ironically, "Bad Bad Leroy Brown". This song along is not rhythmic -- Croce lets the background music carry the rhythm, and he wraps the words around the rhythm is a non-rhythmic way, as many singers do. The irony, for me, is that even though this is Croce's only famous fast song, I do not like this song, I never did, and I never knew why. I do like all of the other fast songs on Croce's "Best of" album. The irony then is that, at least from my view, Croce is famous for a song that is not very good and which does not correctly portray his ability as a singer.
His slow songs tend to be nonrhythmic, but the story can be more complex. "Time in a Bottle", has a beautiful contrast between the rhythmic styles. The verses are nonrhythmic, but the chorus, in addition to switching toa major key, is much more rhythmic.
Other Musical Effects
None known. I know of no use of "hard" voice. I know of no use of "breaks" in voice. Roughly, his voice is always the same.
It is difficult to pick out best single moments in Croce songs. Perhaps one good moment, though well-known, is the shift from verse to chorus in "Time in a Bottle". As noted above, Croce changes to a major key and also becomes more rhythmic. (Curiously, this contradicts the words. The verses are about beautiful possibilities; the chorus, despite its upbeat sound, laments that the beautiful things won't happen.)
The only moment that I like and can't explain is on "genious" in "Car Wash Blues". My memory was that both syllables of this note are song on the same pitch. The fact is, there is a serious drop in pitch on the second syllable. I don't understand what the correct note is supposed to be; if it is a drop, it is the only drop I know of that Croce sings, and as such it is much larger than any other drop Croce does -- as noted, he consistent does small drops. I am guessing it is not just a drop, but then I don't know what it is.
The same song has the line "90 days for nonsupport." The pitch is the same for nine, ty, days, non, and port. As a general principle, repetition of pitch should usually be made more interesting by given them different glides. Croce does this (and perhaps makes the glides a little larger than usual).
I worked from Jim Croce: Photographs and Memories, His Greatest Hits.