First, I should define what I consider to be 'old' VNs. I essentially define 'old' VNs by the 'ten year rule'. When ten years have passed, generally the cultural references, the artwork, and even the sound styles have changed enough to be almost completely distinct from the most modern VNs. At present, that means VNs made before 2007.
Now, next I need to make a statement... I am not an art bigot. One of the most negative issues I've run across in dealing with newer VN readers is art bigotry. To be blunt, there are lots of people who won't read anything made before 2010 simply because the art style is so different. To those people I say... 'every era has its own taste'. While VN art has indeed gotten more refined in the ten years, to the point where it has gotten to where you hardly even notice the characters are drawn in the first place, I can straight-out say that quality art is quality art, regardless of the era.
Now for sound... setting aside music, which really hasn't been refined at all in the last ten years (if anything, it has regressed, especially usage), voice-acting and sound effects have actually evolved a great deal in the last ten years... at least to the point that you are less likely run across the 'comical' sound effects common in a large portion of VNs ten years ago. Voice acting has mostly evolved in the sense that people that once would have become pros don't make it anymore, so the industry has become higher cost (for the developers) and higher quality (for the consumers). In that sense, I can understand some degree of prejudice.
However, when it comes down to it... I'm a story addict. Yes, I became an otaku because of the way the Japanese treated animation art. However, it is the stories that have kept me going. Now, in my less than copious spare time, I've been re-reading some old VNs... and I've noticed a few things I probably would have missed a few years ago.
1. Slice-of-life was less oppressive ten years ago- I don't think I would have realized this if I hadn't taken this little trip to the past, but the excessively long slice-of-life scenes that define modern charage have been getting longer and longer per scene with every year. Part of this is probably because of the nostalgia quotient rising for the long-time otakus in comparison with how it was previously. However, it is a poisonous trend that is actually making the experience less pleasant and more tedious as time passes, unfortunately.
2. A well-drawn line can be as pretty as any hyper-quality modern artwork if done right- This is something I always asserted in private conversations, but I wasn't sure if it was pure nostalgia until I went back and actually re-experienced a few old VNs. Yes, the styles were somewhat cruder back then... but the aesthetics were, if anything, more distinct and beautiful in and of themselves.
3. Ero was weaker... except when it wasn't - To be blunt, the emphasis placed on erotic content and the effort put into it was far lower in non-nukige VNs ten years ago. Less interest went into making heroines more erotic and more was put into making situations erotic, probably to let the libido-poisoned brains of the average male actually look at the characters before they saw them naked, rather than focusing on projection oppai. That isn't to say the erotic situations weren't erotic... but there was a far stronger emotional element involved because of the way they handled the character designs outside of h-scenes.
4. A good story might age badly, but the ones that don't, don't- Some VNs lose all their attraction as they age and more modern VNs exceed them in every possible way. However, there are still gems out there that are as awesome now as they were the day they were made. Rejecting VNs simply because they are old is a short-sighted approach that makes me feel nothing but contempt, after my experiences of the last few months.