ILs and DPs are at opposite ends of the irritation scale:
PIL are obsessed by eating vegetables, five with every meal, never mind five a day. Prior to cooking, they must always have a detailed conversation about exactly how long they need to be cooked for. My DM thinks that of a vegetable is still recognisable in its original form, it is undercooked.
PIL house is always cold, despite having not one ounce of fat between them due to their super healthy diet. My parents have an aga and an open fire in the same room, making it just slightly hotter than the sun. It drives DH potty and he ends up stripping off and sitting at one end of the room with the door open.
PIL have a no flush before 8am rule. Drives me mad, as invariably one of them will have used the loo in the night, and being pregnant I am always desperate to go when DD wakes me up at 6.
PIL are not early risers meaning that entertaining DD for 4 hours in the morning is challenging. My parents wake up at the crack of dawn, and frequently make arrangements for us to set off somewhere at 8 or 8.30, and can't possibly understand why we wouldn't want to make the most of our holiday in this way. I can cope with this, for DH it is more of a challenge.
PIL house is locked up at night like Fort Knox. All doors, including internal doors are locked, and bizarrely the keys are then swapped around, so in the morning, you have to go searching and trying out random keys from other doors to gain access to the room of your choice.
PIL also have rather lax standards of food safety. On the hottest day of the year last year they left a joint of meat and bowl of coleslaw from lunch out uncovered on their dining table. I was curious as to how long it would be left for. Needless to say it was still there when they served it up for lunch the next day. Same with the turkey at Christmas. Fortunately I managed to avoid looking rude at boxing day lunch as DH proclaimed that neither he, DD or myself would be eating it.