I think I can only support what's been said here this far - well, maybe except for
Morthoron's choice with whom to party (and I would also vehemently protest against "colorless bunch", at least when it comes to Ulmo, but that's another topic
), because indeed, as
Lindale mentioned, there was this problem with the Greek gods that they were too "chaotic", what more, they were so prone to the negative emotions and stuff, that they could easily choose to pick up a fight with you or curse you if you said something they didn't approve, and if three goddesses came to you to ask which of them you consider the most beautiful, you'd better run for it.
When speaking of the early drafts
Esty mentioned, I think the Valar stood closer to those at the very beginning of their existence and in their early days in Arda, in the sense that they were still learning a lot - I am thinking now about their failures with Melkor (letting him run free in Aman), the gigantic battle before when Utumno was destroyed - they themselves considered it a mistake on their part, or of course Aulë's attempt with the Dwarves (he definitely had something to learn in that event, which he then did), and we could find many more. The Valar became detached only later, slowly, more and more detached from the Middle-Earth as the events were getting closer to the Elves' fading and with the coming of the dominion of Men. So, to be true towards them, they were not that much detached in the sense that they would not care about M-E: they did care, but it just wasn't that much of their business, especially when it came to the Children, because the Children were not in their "domains" (unlike water, plants, whatever), but they were
Ilúvatar's Children (and you can see that when you read the Sil, I think there's something like the Valar thinking about how much they should interfere when the Children of Ilúvatar awakened). And I think it goes further with Men than with Elves, who, after all, could come - and did come - to Aman, and remained in Arda (the realm of Valar, at least at the beginning), but Men's ultimate fate was different and had nothing to do with Valar at all.
Speaking of fate (and getting back to comparing Valar and the Greek gods), you can also see that Valar did not influence people's individual fates as much as the Greek gods did (except for Melkor in the tale of Húrin and his children, for example) - the Greek gods were supposed to have pretty much direct influence over every person's fate. There's nothing like that with Valar, once again I believe for the reason I stated in the paragraph above.
So, overall I would say the strongest similarity would be these "portfolios", sky, sea, nature, whatever; and I think the Greek gods and Valar are similar to an unattentive observer "on first sight", but when one looks deeper, the images seem to part a lot.