The only such feature is that YC alumni usernames are displayed in orange to other alumni. This is one of the two things that HN gives to YC; the other is /jobs.
I'm admittedly a bit sensitive to charges of favouritism on HN because we put so much effort into trying to be even-handed, even though (a) it's impossible to balance these things perfectly, (b) it's hard to measure whether we're getting it right, and (c) some people won't believe what we say anyhow. For what it's worth, the corresponding complaint on the other side (i.e. among YC founders) is that HN is biased against them.
I'm admittedly a bit sensitive to charges of favouritism on HN because we put so much effort into trying to be even-handed, even though (a) it's impossible to balance these things perfectly, (b) it's hard to measure whether we're getting it right, and (c) some people won't believe what we say anyhow. For what it's worth, the corresponding complaint on the other side (i.e. among YC founders) is that HN is biased against them.