Minecraft is known for its blocks and buildings; and because of this, the architecture of buildings and cities are going to be quite absurd.
Shelter
A house that in real life needs about 5 months to build. A house can be built in Minecraft within a few minutes. Safe to say that, unless overpopulation peaks, homelessness would be virtually impossible; unless the Steve intends to be a hermit or nomad. Since sanitation is not an issue; homes can easily be made as holes in the ground, even if you are just going out camping.
At the same time; destroying a home and stealing from it is very easy since even the hardest blocks can be picked or chopped away at with tools. You can make a nice palace made out of cakes; but a greedy thief, raiding Mongolian horde or a basement troll would chop it apart with ease while you are chilling at Starbucks. Obsidian houses are a possibility, but are more tedious to construct and would be considered more valuable; and also can still be torn down with tools. The majority of any civilization are low income, so not everyone will have access to the best resources or building materials. Wood planks are arguable the easiest blocks to use for construction; and it's not very strong in Minecraft.
A police force would have to be a necessity in any city, regardless of the size. As mentioned before; prisons would have to be very complex in order to prevent someone from easily digging their way out. The only simple deterrent would be to make arson and theft be a severe felony. The Middle East has been criticized for still maintaining the punishment of 'cutting off the hands of a thief' (aka why Jasmine from Aladdin was threatened with a sword for stealing an apple). However it is exactly this harshness that maintains the deterrent. Many criminologists cite pacifism as a major cause of the massive prison population in the USA; some gangsters do not care if they are thrown in prison, they don't consider that a threat. But at least there are locks and alarms in the USA to prevent robberies; most Minecraft homes will not have any alarm or defense against thieves (unless they make a redstone contraption; but not everyone is going to make or afford it.) Watchtowers could exist for the smaller towns.
If a city is stable enough to repress this crime, then we can proceed with construction of these two things; arts and architecture.
Art
As expected; everything will be blocky. But otherwise; art will be basically anything you can think of. It would be difficult to make any detailed piece of art without making it the size of Godzilla, but it would also be easy to do. Most likely the biomes considered to be irrelevant in resources (like oceans, swamps and stone mountains; Nether and End might be too dangerous…) would be used to house these art projects. Again; they would be major targets for terrorists, but it would be interesting nonetheless what massive structures would be made.
City Architecture
Cities will be both practical and impossibly amazing. The richest cities would look like a utopia beyond physical possibilities; with palaces potentially paved with gold, diamonds or emeralds. Most large multiplayer servers already have these locations, so I won't mention them much here.
Highways will exist in some cities; especially highways that go above forests or oceans or drill through mountains. These highways can be for either monorails/railroads (via minecarts) or for walking/horse riding. Whether private highways are allowed to be constructed or if these roads will have tolls will depend on the city. However roads have historically been targets of bandits due to their traffic yet lack of security; think coach and train robberies of the Wild West. A policeforce would have to be made specifically to patrol the roads; especially for monorails which could be sabotaged and redirected into lavatraps. Caravans would be common, unless a monorail is created for 'Minecart with Chest's.
Would hospitals even be a thing? Considering that milk cures poisons and food heals; I would think a farm would be a hospital. Even then, the demand would not be significant considering how easy it is to get food. The only significant aliments would be famine and a potential Zombie Plague: considering the latter, it is possible that police might carry golden apples for the sake of curing Zombies. Heck; the police can be medics as well to assist anyone who is harmed by feeding them. Cemeteries would not exist (in the sense of burying bodies), but obituaries would exist in some form; including tombstones.
Plumbing and sewage would also be irrelevant. 'Lava Trashcans' are frequently used to destroy items, so landfills will most likely not exist. Junkyards maybe could exist, but considering how almost anything can be crafted; it would basically be a Donation Center that will be almost constantly empty. Shops would clearly exist, virtually any multiplayer server has some kind of shop. Blacksmiths would be rare since their skill is universal; although they could still sell their craft.
Carpenters might exist; but considering how ANY Steves of any skill can build, this would be a minimal wage job. Multiple Carpenters would be hired for massive projects, but for house building it would be relatively irrelevant.
Modern cities are clearly full of skyscrapers, and this can be taken to extremes in Minecraft. Keep in mind that skyscrapers began as recently as the 1800s; but such a concept in Minecraft will exist since the dawn of mankind. Pyramids would either be seen as lazy, or would be literally the size of a mountain.
Hostile Mob Zoos are a possibility, if the cages are shaded. Although I don't think people would enjoy seeing the animals that maul them on a weekly basis.
Terraforming
Tragedy of the Commons is a concept that, without strict regulations, natural resources would eventually be stripped regardless of the damage that would do to society. Some cities will be surrounded by wastelands caused by greedy civilians over-mining the land. As mentioned before; oceans are a relatively minor biome for resources; so they could be drained for the sake of tapping into the dirt resources underneath to protect the land. Renewable blocks, like Wooden Planks, could replace the independent quarries so that there is still ground to walk on; but boy, that would be sad to see that the very ground had to be artificially replaced because Steves got greedy. Maybe just be lazy and settle your town ontop of bedrock to guarantee that you have foundation to walk on.
Creative Mode
Basically the Twilight Zone episode A Nice Place to Visit: you can do basically anything and you can't die, and despite being able to make planets out of diamond, it would get very boring very fast. So enjoy divine immortality, then eventually, jump into the void!