I thought it was OK as well. I think people were mad because it really had a lot going for it as far as hype goes. I mean, it came right after the most acclaimed Bond movie in decades; not only was Blofeld coming back but Christoph Waltz was playing him; and the trailers looked cool. It is kind of the Dark Knight Rises of the recent Bond movies if you will. It kind of has the same strengths and weaknesses. Strong opening, inspired casting for its villain(s), technically well made...not much happening by the second act and the central evil scheme is underwhelming compared to the long running time.
I don't think it's 'hated' but it just seems to fly-off-the-radar due to Craig's other two more praised movies, Casino Royale/Skyfall, which are among My Top 3 James Bond movies in general, which are...
1.Skyfall (2012) - 9/10 (within my Top 16 movies in general) 2.Casino Royale (2006) - 8/10 (within my Top 40 movies in general) 3.Spectre (2015) - 7.5-8/10 (within my Top 80 movies in general)
plus, I think some of what people may not like about it is it's more like a traditional Bond movie unlike the rest of the Craig Bond movies which are not.
EDIT: adjusted some info above so it's accurate as of Aug 16th 2019.
I do think that Casino Royale and Skyfall were both way better movies. However, out of the Craig era, Spectre felt the most like a traditional James Bond film to me.
While I do agree Casino Royale and Skyfall are better than Spectre, I don't think they are 'way better'. but I totally agree with you on your 'traditional James Bond film' comment as it feels the most like a traditional Bond movie of the Craig era as I think that's pretty clear cut.
The number one thing for me was that I felt it was slow and lacked any great action setpieces, but I also really hated the ending. Bond leaving the life to go off and marry a woman he barely knew kind of betrayed Bond's arc in Casino Royale. Bond may achieve double-o status in the beginning of that movie, but he becomes Bond at the end with the line "The mission is over, the bitch is dead", the heartbreak of Vesper's betrayal and dying in front of his eyes should've been this incarnation of Bond's total hardening.