with so many people in one place - possible language barriers, sanitation, problems with spread of diseases is possible (but im not sure its too much of an issue nowadays as Saudi authorities are really good)
maybe getting lost in the crowd, or getting crushed with so many people (again i think this only happened when they would carry out teh stoning of the Devil - but they have changed this so i dont think there are many deaths here due to that)
Accomodation caused a lot of deaths in the past - but again the authorities have changed this as well so only new buildings.
mina tents used to catch on fire - but now they have made these fireproof
i guess they've covered all the bases:sunny:
ummm ok it is physically challenging for the elderly or disabled (although wheelchairs etc are provided) but the lack of sleep and long journey time will take its toll. Money wise Hajj costs alot an would be harder for poorer muslims to perform. hope this helps