The humor, and particularly Stabby Mike. I have played so many other visual novels since this one that it's a bit blurred in my mind now. What I remember is some extreme suspense early on when the MC walks around in a church where he has no business and a bunch of seriously secretive and apparently dangerous people are likely to catch him. And chaotic Stabby Mike appearing again and again in the most unlikely situations.
Eternum is a similar blur for me. It's a good thing because I can enjoy it again and again almost like the first time. But Eternum isn't quite as funny, and the suspense is never quite as extreme as in the early Once in a Lifetime.
I guess the reasons why a lot of people prefer Eternum are 1) Caribdis' graphics are now just as ingenious as before but with much improved rendering quality, and 2) the science fiction / fantasy setting that blurs reality and virtual reality appeals to a lot of people.