names - Why are parshat Noach and parshat Toldot named that way?

I was taught that the parshiyot are named for the first significant word (or, rarely, phrase) in the parsha, and obviously names have to be unique. This week's parsha begins "eileh toldot Noach" and is named "Noach". Later we'll get one that begins "eileh toldot Yitzchak" and is named... "Toldot", not "Yitzchak".

Why is this? Does the later one, being about a patriarch, claim the earlier word "toldot"? Is it because the entire Noach story is contained in the earlier parsha so we name it after him? Is there some other reason?

Answer

In a number of his talks, the Lubavitcher Rebbe zt"l points out that this demonstrates that the names of the parshiyos aren't just incipits, but reflect the theme of the parsha (and that this is generally true of the Jewish names of people and things).

The difference is basically this: Parshas Noach is primarily about Noach himself, not so much about his children (whose birth is already mentioned at the end of the previous parshah), so it's more appropriately called by his name. Conversely, Parshas Toldos is not so much about Yitzchak himself - many of the major events of his life have already been discussed in previous parshiyos; here the focus is more on his children ("toldos"), Yaakov and Eisav.

(Likkutei Sichos, vol. 5, pp. 354ff; ibid., vol. 25, pp. 126ff; Sefer Hasichos 5752, vol. 1, pp. 114ff)