Yeah, I think it boils down to just how tall the frets are in relation to how high the strings are above the fingerboard.  The shallower the frets, the more variation you can get in the pitch between frets since there will be room for the string to float just above a fret without touching it if you're fingering off the position.  Also, in the higher ranges where the frets are closer together, they'd really need to be even more shallow, but in that range, just the difference between fingering on the front of the fret and the back of the fret yields a noticeable pitch difference.  Since you're fingering right on the fret unlike a guitar, this is quite useable.

You look at a guy like U Srinivas, who modified the 5-string electric mandolin, he made similar changes to the fret layout, and he plays vibrato with slides alone, never pulling.  In this version where he's a lot younger, he hadn't yet shaved down the frets to the extent he does nowadays (this video is probably ~30 years old), but even here, he's not pulling.

I do find, though, that pretty shallow frets are more than adequate for the violin.  I happen to keep 3 finger tapes on my board -- just perfect 4th, just perfect 5th, and octave.  I derive every other position from those.  The thing is that simple tape strips are far thinner than any fret, but they're still not flush with the fingerboard, and that thickness is enough for me to get a tactile feel for their presence and catch the note with my eyes shut.

@Daniel : I agree about the straps.  I found about the Wood Viper some time back, and I've always found that strap with the chest pad thing.  It pretty much solves so many problems.  I've been working out some designs on paper to try and do something like that for a regular acoustic as well, but it takes some trial and error.

I seriously wish I could afford one of those things, but I got a quote for a 4-string fretless with the "default no-extra-charge" finish and the Wood standard pickup (is it just me, or do other people find the Barbera pickup a little rubbery-sounding?), and it still ran about $1885.  Adding frets brought it up over $2k, but I also was trying to inquire about fretting according to alternate intonations.  I might be willing to spend that kind of scratch when I have about double the cash in the bank as I currently do and about 20x the skill that I currently have, but not before.