Let me give some objective reasons I found this song dull (objective in that they exist - not whether they are a good or bad thing) 1) No changes - It's the same chord progression all the way through 2) Undynamic arrangement - The music never goes anywhere - her voice stays mostly in the same range and the other instruments stay at the same level/density. Here are some more subjective reasons 1) Dull melody - It's not particularly memorable 2) Been there, done that - It sounds like a million other ballads - there's nothing to distinguish it. She has a nice voice but I agree that she needs to work with some better songwriters (and producers for that matter).
Someone forwarded me a video of her singing Chasing Pavements a while back - it was ok, but nothing that made me say "wow!". The same is true with this song. I think she has a nice voice, but the songs I've heard her singing don't really do anything for me. It doesn't really evoke anything from me like when I first heard Mariah Carey's voice on her first album, Anita Baker when she sings anything, or Alicia Keys' Fallin' video when I first saw it, Etta James singing "I'd Rather Go Blind", and so many others that when I hear it, I want to hear the voice even more and/or pay close attention to the lyrics (which is rare for me).
I guess I like the song for the same reasons people didn't like it. It captured a very specific atmosphere/mood and did it well. Almost like a soundtrack vibe. I prefer songs that ebb and flow, which explains my preference for certain electronic/dance music.
I freaking love this song. She reminds me a little bit of Cass Elliot with a better voice. Awesome. Just awesome.