For me (in order of importance), it’s:
1.) Is the bike and it’s builder truly special, unique and/or important. Is it “hand-made”? Pretty much all mass-market bikes (meaning, built by the hundreds, thousands or more in large, often Asian factories) are immediately eliminated on this point. My favorites would be those designed and built by a single, known individual or tiny “factory” operation.
2.) Is the bike either well preserved or well restored? Was attention paid to the details? Are exceptions explained or justified?
3.) Personal taste. I don’t care for suspension (front or rear) and I hate anodized parts in any color but silver/clear or black. (Thankfully largely absent in the 1980’s and hence this month’s entries)