There are actually three sentences worth discussing
(1) あなたはやさしい人です
(2) あなたはやさしい人だ
(3) あなたはやさしい人
For each sentence, we should consider three dimensions: grammaticality, softness, and politeness.
(1) is soft (doesn't sound too direct), polite (shows respect for addressee).
(2) is rough (sounds like a point's trying to be made), not polite (doesn't show respect for addressee).
(3) is soft (doesn't sound too direct), not polite (doesn't show respect for addressee).
All three are grammatical.
(Note: "Not polite" doesn't necessarily mean "rude". There is no need to use the polite form with your friends, close family, or people much younger than you, for example -- in these cases the polite form would be overly distant or odd.)
(Note 2: It's probably worth pointing out that the presence of あなた makes all of these sentences sound a little jarring.)