All aquariums needs lots of patience.  Reef tanks, however, will try a man's soul.

I'd say most of the aquarium problems I help out with are due to two things.  

1)  Lack of patience
2)  Trying to "cure" things because of #1

If you know why things need to be the way they should (acceptable ranges, etc), keeping aquariums is so much easier.   Just trying to do step by step instructions without knowing the rhyme or reason behind it will result in an expensive nightmare.

I'm saying this as a guy who just nuked his larger display tank by accidentally getting veterinary grade 'safe' disinfectant in the water when cleaning up after my parrot.   And I was very careful.  

Another rule to remember:  The first batch of fish is disposable, the most boring one will outlive everything else, though.

Difficulty, as well as cost, after initial setup, larger the tank the better because Dilution is the Solution to Pollution.
Fake Plant freshwater
Planted Natural freshwater (closed cycle, uses real dirt, etc)
CO₂ planted freshwater, high tech LED lighting, very awesome looking when done right (see "aquascape")
Fish Only with Live Rock Saltwater (FOWLR)   -  Simplest saltwater.  Good Lights needed.
Reef Tank - Full on ocean conditions - Specialized lighting and circulation needed, preferably with a quarter sized sump so the display is pristine
Nano Reef - everything of a Reef, in 10 gallons or less. (lower cost, way higher tech skills)

--ETA:   One thing you Won't Regret:  A Good RO system.  Making your own water, no matter what the end use is, guarantees happier fish.   RO/DI water + Seachem products and you are golden.   Only re-add GH/KH for freshwater, you can make it as soft or hard as you like by adjusting the GH.     Just keep in mind that you are starting out with 0ppm water.  It will kill fish, unless you add in the needed electrolytes (what fishies crave!).    Freshwater Fish "drink" through osmosis, so if you have 0ppm water, you are dehydrating them.   Saltwater fish drink like Humans, and have a lower concentration of electrolytes in them than in the water they are in, the opposite of freshwater.