I have caught Hitman too late to play H1 (later I read reviews, which mentioned some bugs and gameplay slowdowns and I didn´t want to spoil great H2 experience). I played H2 and it was one of the best games of the year. You're right with sneak-in problems with snipers in getting-to-the-Japanese-castle level. Maybe you acted in inapropriate way, got off the road, maybe they had an order to shoot anyone approaching the castle, I don't know. And to be honest, I simply shot them and hadn't let them to destroy the great feeling. But it wasn't general problem of the game, most of the levels (eg. just the next one in the castle) were playable with no shot, or only a one or two shots fired...
I'm playing Contracts now and to me, it seems that the gameplay hasn't changed much from H2. But not sure, it is a long time ;-)