"The land will therefore no longer continue to yield its strength to Kayin". What exactly did that entail?
Ramban: Whether Kayin plowed, dug or planted, he would never achieve what he expected to. For example, he would plant a lot, but only a little would grow. 1 He would arrive at a pile of grain that had consisted of twenty Sa'ah, but find only ten.
The earth would hide, as it were, all that was planted in it
"You shall be a vagrant and a wanderer on the ground." What has that to do with the first half of the Pasuk?
Rashi: This was an additional curse prohibiting Kayin from remaining for long in any one location.
Why did Hashem mention the punishment for covering Hevel's blood, but not for murdering him?
Seforno: It was not necessary to mention the murder, since everybody knows that someone who causes any form of damage earns himself the same punishment. 1
Radak #1: Kayin was not punished with death, so that the earth would not be desolate of inhabitants. Instead, he was punished with exile, an alternate punishment for murder.
Radak #2: Kayin was deemed inadvertent, for he could not know that the blow he gave Hevel would be sufficient to kill him. He was therefore punished with exile.
As the Torah writes in Parshas Emor (Vayikra 24:19). This is unclear however, since Kayin was not sentenced to death, as the Pasuk there implies. Refer to 4:14:2:1. Perhaps Seforno is referring to the killing of Kayin seven generations later, as Rashi (to 4:15) explains, or because Galus is akin to death - Refer to 4:13:1:3.
QUESTIONS ON RASHI
Rashi writes: "'You shall be a fugitive wanderer' - You have no permission to settle down in one place." What is Rashi adding?
Gur Aryeh: The Pasuk does not mean that Kayin would be unable to remain in one place due to the curse upon the earth, but rather that Kayin should willingly take upon himself exile, due to his sin.