The problem is that a bunch of other straightforward old school SHMUPs that came out after it were already better than Darius (Raiden and Star Soldier series comes to mind). Also, compared to R-Type, Gradius and a bunch of others, there's just not much going on for Darius. To me, it just didn't age that well.
Oh, and the level branching isn't really that innovative when all you do is change the scenery but keep the same repetitive enemy waves over and over again. I must say that the 3 screens arcades looked nice though, but yeah, I wasn't born when that happened.