What's your favourite OELVN?
One Thousand Lies
Will OELVNs ever be as good as Japanese VNs?
No...
Why are OELVNs generally considered worse than Japanese VNs?
Because they are, 90% of the time. Yes, there is good OELVNs out there, (like One Thousand Lies), but it's quite rare to find them.
Another issue is that most of them are made by indie developers with very small budgets, making it look rather "amateurish". I hate to say it, but the truth is, even the most amazingly written game can still look bad to most onlookers compared to the new, shiny, high budget VNs you can play instead. I think that's the biggest issue with OELVNs trying to make it in this market. And of course, the high amount of crap. (Sakura games and what not is a thing after all.)
To clearify, I'm not saying art is the most important thing in games, in fact it probably is the least important when you think about it, but first impressions is a real thing, and when you compare something with amazingly fresh and stunning art to something that looks like it was made in paint, people will naturally think the one with nicer art is better, even if it actually is bad in all the other aspects.
What are the attributes that make OELVNs stand out (in a positive way) from their Japanese counterparts?
They are usually written very differently, and this makes for a cool, new experience.
Recommend a random (relatively unknown) OELVN
I'm just going to recommned One Thousand Lies, because it's really good, and everyone should play it. (And it's free!)