A few ideas about the sins that cost Moshe's generation a chance to conquer the land of Israel:
- Ramban focuses on the word אפס (translation: but, BaMidbar 13:28). If this was in fact a factual report, there was no need for a qualifier - stating facts was all that was required. 'But' was a contradiction to their first two sentences, that described the land so glowingly.
- Why was there a need to mention Amalek (BaMidbar 13:29)? Amalek was deliberately mentioned, despite not even being part of the land of Israel, in order to incite fear and doubt.
- Rav Twerski points out that the Spies' report highlighed their lack of belief in the mission at hand: "We were like grasshoppers ... and so we were in their eyes," (BaMidbar 13:33). So lacking in confidence were the Spies that they felt 'small' during the mission, and even anticipated how the residents of the land would view them. Hashem had taken Israel out of Egypt, but their feelings of weakness and inferiority due to the years of slavery had sadly accompanied them. This generation wouldn't have the privilege of conquering the land.
- לא נוכל לעלות (BaMidbar, 13:31) ... The Spies tried to convince the Nation of Israel that the task was not possible ("We cannot ascend"). The mass hysteria that followed on Tisha Be'Av ("the people wept that night," BaMidbar 14:1) has haunted us till this very day.