Well here's my tuppence, and my vote was to only disable TNT.
Water - Not often used for grieving, don't think its a great threat and even if it was used as one it is pretty easy to fix with a stack of dirt! People like to use it to make things look nice and they should be allowed to, also allowed to be used to for any type of farms which come in really useful very often.
Lava - Well it is a bit of a threat regarding grieving, but it's fairly easily fixed if you think about it, grab a stack of dirt and hit the sources, and when you have to run through lava it is slightly annoying but you have infinite health so there isn't much to complain about. People like to use it to make things that look cool and they should be allowed to, also it is used on obsidian farms which is really useful sometimes.
Fire - Not really too annoying, can burn wood in places and is non-trackable but I think that it really should not be disabled because it is a fair threat of grieving but can be used for pretty awesome things like fireplaces which I like to make
TNT - No need for it, quite hard to get but there is always the chance of hacking/duping and someone actually so determined to make enough TNT on the build server to grieve somewhere. No one really uses it and when they do its often used for grieving. As xanderh said it should be disabled to avoid grieving that bit more. I personally think this is the only one that should be disabled because there is no need for this and quite a threat of grieving.
TIP
For a fire place: it looks best on a stone floor with stone walls and a stone roof 3 blocks high. First get a log of wood and place it down, directly above the wood dig up the roof 2 more blocks; use 8 blocks of cobble to surround it on either side and on the corners as well. put glass on top of the cobble and another layer of cobble on-top of this glass. Set the log on fire and the fireplace should look great. I like to put a sofa and chairs near this as well just for the looks!
