I am probably very old-fashioned, but I think that for Royal Weddings it is a bit discouretous for guests to wear large and/ or unusual hats. Smaller hats [I didn't say 'small' hats, I just mean 'regular-sized hats'] to me seem to the way forward in my very humble opinion. This is because large hats - although they can look glorious - do tend to impact on the views of others. It doesn't matter so much at smaller weddings because (for example) If you find yourself sitting behind a guest with a Beatrice-type monstrosity, you can usually 'position' yourself slightly so that your view of proceedings is no longer obscured; at a Royal Wedding [or other 'Important' wedding] space is usually at a premium and so you can't move. William and Catherine's wedding was a little unusual for the 'ordinary' guests, if I can use that word - i.e. I mean those who were not 'friends / family' because one of the guests yesterday [the Chief Executive of Centre Point from memory] revealed that his admission card placed him only in a specific 'Area', not a specific seat, and so within this defined area, you could 'seat yourself'. Most important Official Chuch services I have attended have always included an assigned seat number, so if I find yourself sitting behind someone with a towerering hat........I just have to grin and bear it.
With regard to Beatrice's hat, I see from the papers that some 'fashion-istas' are saying that her sartorial mishap was the direct result of not having a mother present to say 'Eeeek, you can't wear that for a Royal Wedding, think of everyone else who has to see round you'. Mind you, my response to that was that the designer who created it [Philip Treacy or have I misremembered?] as a couture milliner should have reminded her himself.
Incidentally, I have the honour to be in the Royal Enclosure at Royal Ascot each year, and if you turn up in a hat that is likely to obscure people's views too much, the officials remind you that it is a breach of good manners and won't let you into the Royal Enclosure, only the Paddock and Grandstand.
Alex