Not read whole thread but
Long boring speeches where an endless parade of rellies to bore on for half an hour. You think you've sat through them all, but no, now it's time for Uncle Bert's stepdaughter's cat to make yet another identikit speech. 
Handshaking lines - please no, so awkward and tiresome.
Being sat with strangers at your table when you have perfectly good friends who you'd like to talk to. Why mix us all up? Put me with my mates.
No-child weddings - yes it's your choice but they are a bit joyless to me. But hey ho I probably can't come anyway!
Badly organised catering where seems to take 2 hours to bring the first course out. And why is my table always the last? It's better to have a buffet than really slow posh catering.
Endless photo sessions, especially when you're not part of the main wedding party but are still required to wait for 3 hours because at some point you will be required for "bride and her mates all leaping in the air with happy grins". Just have an informal photographer taking pics of the day and ask people to send you their best cameraphone shots, they'll be just as good. DP took an amazing pic at a friend's recent wedding, which they loved, and he's not a photographer at all.
Vicars who use the opportunity to guilt non-christians who have made a rare visit to church. Well of course we have it's a friend's wedding. I do not want fire and brimstone thanks.
Having said all this I actually love weddings! What I love - nice food, short speeches, lots of music and dancing (live band is great, also cheesy nostalgic hits), informality, fun extras - one couple we know had a table of amazing mini cakes and pastries with tea, as a mid-afternoon snack after the ceremony as there was a wait before the meal. I like it when it is obviously the bride and groom's own do and they and their friends get to have fun. I like it less when the parents and rellies take over and it's all very stuffy and dull and there are 50 long-lost great aunts and cousins who nobody knows (not even the B&G).