One tiny event, something as simple as who moves out of whose way while going down stairs, can make all the difference. With Sliding Doors we get to see both timelines. This romantic comedy gives us twice the lack of romance and twice the lack of comedy for the same set of mundane characters dealing with the same basic set of uninteresting mistakes.
One of Pixar's rules of story writing amounts to: it's ok to set up the plot with a coincidence, but not to resolve it with one. This movie elaborates on that: especially to wedge in a happy ending where there isn't one.
Gwenneth Paltrow does a fine job with what role she's given. It paid her bills.