Your biggest challenge for a reef tank that size will be lighting. Lighting for any size tank is expensive as it is. Lighting for a big tank is "I'll trade you my firstborn child" expensive. I used to have a 240 gallon, and the lighting fixture required to keep it as a reef tank cost nearly $2000. Not to mention the electric bill from running that monster (it was a MH/T5 combo).
Another challenge with a reef tank that size is filling it -- lots of rock needed (1 to 2 lbs per gallon). And once you start adding corals, no matter how many corals you put in it, it will still look empty. It takes a lot of $$$ to stock a reef tank with corals, and a tank that size will look barren for a long time if you don't have a lot of corals.
Because of the cost of running a reef that size, large tanks are usually kept as predator tanks. There are tons of awesome fish in this hobby that just can't be kept in reefs because of their dietary habits. If I was to do my 240 over again, I wouldn't do a reef, but I'd do a predator tank instead. Predator tanks do not require any special lighting and for the most part, you don't have to worry about corals.
