I'll say something about it while I drink my coffee here, but keep in mind this opinion is subject to my own tastes:
Liquid originally spun out of a House music and wasn't really defined well until Hospital Records started up and really began to nail down the signature sound that liquid came to be largely identified as. That really shaped the contemporary image of what liquid is, since the new "kings" of liquid such as Netsky and Fred V were influenced primarily by Hospital. They in turn spawned a generation of producers suffering from the impression that you just have to be born in Europe in a house with a computer with an internet connection to make awesome liquid choonz!
Anyway, I digress. House itself had different flavours which it lent to the modern liquid soundscape, so the evolution of liquid really harkens back even further to the big band sounds that influenced soul and funk, which in turn influenced disco music, subsequently influencing house music and ultimately liquid. Because the primordial sounds that shaped all this started with LIVE instruments before computers were even around, it makes sense to classify any dnb that incorporates live string, horn and bass samples as liquid.
For people like myself, who grew up listening to early disco which was heavily influenced by funk/soul, it's important to hear live instrument samples in my liquid. Blaring horns, sweeping string melodies and some bass slap is where it's at! For people who grew up with more modern club influences, on the other hand, the electronica roots are more relevant so those people identify liquid more with euphoric synth melodies rather than soul and funk samples. It all depends on where your own musical influences originate from, but as far as liquid funk goes these two sounds are the ones that, for the most part, make the genre.
I say "for the most part" because there's a resurgence lately of the happy hardcore influence which plays well at the club with the euphoric liquid. Take
this recent release as an example of that. Happy New Year to the candy raver in you
