Regarding the other sons, the Torah gives the reason before the name. Why is Reuven different?
Rashi and Moshav Zekenim: His name is put first to praise him - 'See the difference between my son, whose Bechorah will be taken from him and given to Yosef, 1 and not only will he be silent, but he will even plan to take him out of the pit, 2 and (the son of my father-in-law) Esav, who wanted to kill Yaakov for taking away his Bechorah, even though he himself had sold it to him' (Berachos 7b). 3
Oznayim la'Torah: From the fact that Leah knew all this, it is clear that Leah was a Nevi'ah. Refer also to 29:34:151.1.
Divrei Eliyahu, Kol Eliyahu, Ha'amek Davar: R. Eliezer expounded so. Why does he expound unlike the Torah's reason?! Regarding all the others, it gives the reason before the name. For Reuven, it says the name first. This shows that there was a reason to call him Reuven other than the reason written.
What is "b'Onyi"?
Tosfos ha'Shalem (5, citing R. Efrayim): (It is withholding Onah, intimacy at the proper times.) Yaakov does not even fulfill [with me] the Onah of a Chacham (Shabbos night). 1
Perhaps she could accept that Yaakov prefers Rachel, but even when Rachel was Nidah, Yaakov would not fulfill Onas Chacham with Leah. (PF)