Pros of having an autumn wedding:
Weather: The weather in autumn months is maybe slightly more reliable than the other seasons, at least here in the UK anyway. The temperatures are starting to cool, which can make for a much more comfortable experience than in the height of the summer. Also, the days are still long enough to ensure that if you wanted to, you could have an outdoor ceremony, outdoor cocktail hour and all of your photos outside too!
Natural colour scheme: Autumn brings with it a special set of colours that perfectly match what nature has on display. If you are looking for a deeper colour palette, with rich oranges, bronzes, purples and reds, then autumn is definitely the season for you.
Theme: If you are looking for a rustic barn style wedding, or a shabby-chic or rustic-chic look, then autumn is the perfect season to showcase this theme to perfection.
Cons of having an autumn wedding:
Expense: I don't mean this in terms of you paying for your wedding, but in terms of your guests. Your guests may still be paying off their summer holiday and trying to save some pennies for Christmas presents. Just be aware that money can be tight for many people at this time of year.
Venue: If you dream of having an outdoor ceremony, then you had better go for September for your autumn wedding. As the days get shorter and colder, you are less likely to want to be outside for long periods of time (especially wearing a wedding dress and no coat!). Since you will be inside, you are more restricted in where photos can be taken and in your style of decor, depending on your venue.