I think possibly his cancer and the boot syndrome.....

LOL......
Seriously....the the fact that Napoleon liked Artillery so much and he depended on it greatly.....the rain had softened the ground and his artillery wasn't as effective as it had been in previous battles...Wellington didn't like artillery and he relied onit very little so it had very little consequence to him.
Personally I love Artillery and it was a great weapon for its time.....
WELLINGTON
Wellington I think your a little confused about the fact that wellington didn,t like artillary any good general of the time or even a bad one for that matter could see the importance of artillary on any field of battle and it was a very important and munch needed tool and was the cause of most casaulties. I think you may be confusing the fact that wellington hated rockets with artillary. Rockets were a new form of artillary that were very unaccurate and at times could take out as many of their own men as their foes!
Wellington used his artillary very well at waterloo and had a major advantage over the french in the form of spherical case shot which was a hollow canon ball filled with shrapnel which had a fuse and would go off in the air showering the field with smaller shot and taking out many troops. Many of the french attacking hougomont fell to this form of shot and this kind of shot would also not slow and get stuck in the mud as it went off before hitting the ground if the fuse was set right. This was a secret weapon of the british at the time and not used by other nations.
As to the one thing that cost napoleon the battle of waterloo it had to be hands down the arrival of the prussains but you could make a very good sized list of things that also added up to costing him the battle.
One would be the delay of starting the battle for the field to dry that allowed the prussains to arrive in time which was a mistake because the field would have taken a day or two to dry to improve. I think the delayed arrival of the gaurd units to the battle field was also a reason for the delay.
The focus on hougomont which was ment to be a deversion to weaken the center was a mistake as it tied up to many troops and I think he should have tried to out flank it to the left and not have made all those funeled attacks between the two farm house stong hold postions or he should have broght in artillary to destroy the walls and weaken the postions.
Wellington feared that napoleon might try to out flank his right and placed troops there out past hougomont which didn,t see munch action, I wonder what the outcome would have been had he tried, but napoleon didn,t seem to put munch effort into the battle plan after it had begun and sat out a lot of the battle leaving Ney in charge. I think a better marshall like davout should have been taken along and grouchy the newest marshall was a bad choice.
The early unsupported cavalry attacks and napoleons refuseal to send in the gaurd at point at which wellingtons center was about to be crushed around the time of the capture of la haie sainte which allowed wellington time to send in critical reserves also all added to the defeat.
There are many more things that added to the defeat but these are some of the main reasons.