Well, mostly it takes money, of which I no longer have...
And commitment to keep up with everything. Water changes, buying salt, knowing what each of the creatures you've bought require in the way of food and vitamins. It doesn't have to be hard...if you stick to things that 1) live off of things in the tank, like my tangs, then you don't have to feed 'em, and 2) if you start buying specialty corals, stick to ones that all require the same additives...they all need calcium, but some require more and that's when you begin to lose track!
I had a 90gal years back, and I got very sick of toting buckets of water around to do water changes, so with this 120gal, my husband, being the designer/builder that he is, had a hole cut in the bottom of the tank, installed pipe and plumbed it in to our H2O system at home...when I do water changes now, I simply open a valve under the tank and dump gallons out in a matter of seconds! Then, pump the replacement salt water in...water changes take about 5 minutes now, and that's helped me heaps with upkeep!
To start a reef tank...
My tank was used and I picked it up for $400
Live rock (base for all reef tanks) $1000
Salt (in bulk) $80
Lights (metal halides and actinics/housing and bulbs) $700
Fans to cool lights $40
Plumbing under tank and sump (filter system) maybe $100
Protein Skimmer $150
R/O system (reverse osmosis - I filter my own water, then add salt) $170
UV filter (kills bad micro-organisms during filtration process) $125
Can't remember what else, but I think I had about $4000 in it by the time I got it all set up, and didn't have a single fish!
It doesn't have to be that pricey though...my tank is huge and it took a lot of live rock to establish it...