Thankfully I haven't been in the situation where I need one but I think having a blood transfusion would be the most difficult. I can imagine their would be intense pressure from my family on that issue as the dubs think there's a medical benefit from abstention as well as a spriritual one.
I haven't had Xmas or a tree yet either, so that's another 'worldly' thing I need to do. I wouldn't find it hard to celebrate Xmas though and put the party hat on and pull the crackers. It's just that all my family are dubs so I'd have no-one to pull my cracker with