I've only been married for one week and one day, but this is something that can easily be observed by anyone, newly married, single, or married for years.
In many instances, couples have unrealistic expectations about their lives and their partners after they are married. Marriage doesn't change a person's habits, personality traits, opinions, beliefs, etc. If a person chews with their mouth open before they get married, they are likely to chew with their mouth open after they get married, even if it annoys their significant other. Getting married doesn't change a person.
This isn't to say that taking those vows doesn't make a person strive to be better in their relationship, but you can't expect a complete 180 degree change of a person. You have to know the person you are in a relationship with, and understand the fact that the things that annoy you about them when you are dating will still be there after you are married, and weigh whether or not this is something you can overlook for a LIFETIME.
I think in many instances by the time the wedding manicure and pedicure polish has faded those quirks and annoyances start to annoy more than they had before. This is only natural, as a person starts to realize they will have to deal with this til death do they part. They key is to think about this BEFORE you make it to the alter. Marriage should be taken seriously and people should truly take the time to think about the worst thing about their partner, and if they love them enough to deal with that worst thing for the rest of their life. In theory, that other person will work on their flaws, but in the instance that it will take them a long time to work on it, can you stay in it for the long haul? If you can...get married.
Marriage is just a grand step toward a lifelong commitment to work on a relationship. The relationship is still going to take work, there will still be arguments, good times and bad times, but the stakes are/should be greater, because it is a lifelong commitment.