Wow, lots of opinions on this one!
Ultimately...yes, I think I would be a fan of Uranus and Neptune even if they weren't a couple, but at the same time them being a couple is definitely a draw for me, and there's nothing wrong with that. I hear what Neon Genesis is saying. Sometimes it does feel like gay characters are examined by different standards from straight characters.
I've been into the Outers before I even knew they existed. Watching Sailor Moon back in 1997 or so, I remember drawing my own versions of Uranus, Neptune, Pluto, and Saturn because as far as we knew back then, they didn't exist (I missed the first appearances of Pluto in R; back then we only had the first 60-something episodes). The fact that the show introduced Moon + the Inners and conspicuously ignored the remaining planets made it seem like these other "missing" Sailors were really special.
I discovered Uranus and Neptune (and, consequently, Pluto and Saturn) on the Internet around 1998 or so and immediately fell in love with them. Their portrayal on the show itself seemed to line up with what I was reading online. They were cooler and more mature than the Inners, and yes, them being in love with one another was a highlight. All of the other characters were devoted solely to Sailor Moon, but Uranus and Neptune were also devoted to one another. That was really special.
But there were other things I liked about them, too. I love how Neptune has ocean-based powers. I like that they have their own weapons/talismans, which the Inners don't really have unless you count their "super" attacks. Uranus and Neptune played heavily into the series mythology regarding Saturn and the overall plot of the S/Infinity arc, whereas the Inners didn't really have this luxury (they existed to protect Usagi and occasionally the 90s anime involved some of them in the main plot more than others, but more often than not they're just experiencing the plot). As people, Haruka and Michiru are kind of fascinating. Haruka rejects society's expectations for her which is quite admirable. She's a famous race car driver and she seems just as rich as Michiru, who is also quite artistic and mystically inclined.
I don't think Michiru is a Mary Sue because she has at least one flaw -- she's so devoted to Haruka that sometimes she doesn't make the smartest decisions. In my mind, the narrative painting her as "perfect" isn't enough to qualify her as a Mary Sue -- she needs to actually BE perfect, and she's not.
Beyond that, I like Michiru's mysterious aura and her overall attitude. It's true what others are saying, that there never really was a Sailor Moon character like her before. At least in Haruka's case we got Makoto -- not the same, to be sure, but I feel like there are some similarities there, whereas Michiru doesn't seem like any of the Inners. She's smart like Ami and she's spiritual like Rei but it's just not that simple with her.
So...yeah, TLDR, I like them and I think I'd like them even if they weren't in a relationship/if they were straight, but that's also part of their appeal and there's nothing wrong with that. I think Neon Genesis is reading a bit too much into it, but at the same time, what they're saying is not entirely inaccurate -- sometimes, it feels like these questions are asked of gay characters quite often, and it can be tiring to point out that it's fine to like a character for being gay.