What are the connotations of "ve'Einenu, ki Lakach oso Elokim"?
Rashi: It means that although he was a Tzadik, Hashem took his life prematurely, because he was prone to become a Rasha. 1
Targum Yonasan: It means that, after living three hundred and sixty-five years here in this world, Hashem took him away and turned him into an angel - Matatron the Great Scribe. 2
Moshav Zekenim #1: He was the seventh generation after Adam, and the seventh is always precious. 3
Moshav Zekenim #2: The Torah writes "ve'Einenu", because he lived much less than the others.
Like we find with regard to a Ben Sorer u'Moreh, about whom Chazal said, 'it is better that he die innocent than guilty' (Sanhedrin 71b).This also explains why he died so much younger than any of the other people listed in this Parshah.
See Oznayim la'Torah, who elaborates on the interpretation of Rashi and of Targum Yonasan, and who prefers that of Targum Yonasan. cites a third interpretation. See also Rosh, Ba'al ha'Turim.
Moshav Zekenim: Similarly, Moshe who was the seventh generation from Avraham, and Eliyahu, who was the seventh generation, both went up to Hashem. (Perhaps this is referring to the Gemara in Bava Basra 121b, which lists seven people whose lives spanned the world, and Eliyahu was the seventh. Perhaps the text should read that David was the seventh son, starting from Eli'av, like it says in Yalkut Shimoni Yisro, 276 - PF).