I remember going from Tiger to Leopard and thinking "this is nice, I like a lot of these changes" and then going from Leopard to Snow Leopard and thinking "wow this is great". Then Lion came along and I was like "this is meh".

So maybe it wasn't peak performance or reliability that caused the feeling, but there was definitely something better about SL, and the reason I know it wasn't just nostalgic rose-tinting is because it was bookended by releases I liked worse.