The sequel to the notoriously tricky and frustrating Siren manages to improve on many aspects, particularly smoother gameplay and better voice acting. The narrative spaghetti of the first game is taken on board with a more refined structure - it doesn't seem as concerned with alternate realities as the first game (or Siren: Blood Curse for that matter). The sightjacking gimmick is also more refined, easier to use and even gives the player chance to control enemies and detect past events.
So why is the first one better? It's hard to pin down but the atmosphere of this game is somewhat different, still well crafted but missing the bizarre elusiveness of the messy first game. I think the key to Siren, for me, is in its mess, the imperfections, the extremely illogical puzzles and overall difficulty - there's a real sense of accomplishment when you are able to piece together narrative strands of that first game. In this one, it's quite easy to take in. Maybe it's the fact that Forbidden Siren 2 is a better game that makes it a lesser Siren game?
That said, the yamibito are much scarier beings and some of the creatures in the third act are pure nightmare fuel!