BAHAHAHA! You're looking for consensus on any issue/element/topic in D&D!?! HAAAAAHAHAHAHA!
Seriously, though, <wipes tear from eye>... come on...
You know the answer to your title query is, "Anything [however "common"] you want or need for the given game/campaign." Right?
Of all the various topics, this could possibly be the one with the broadest swing, because it goes to the very fabric/structure of the setting world...so sits at the very foundational bedrock of the game, itself. So, you're never going to find common agreement among all settings for all gamers.
For me/my homebrew setting:
Magic: Magic is everywhere. But there are different levels of magic in different regions of the setting. You can be somewhere that high level magic hasn't been seen or only exists in their legends of decades to ages ago. You can be elsewhere that high level magic is expected and if you're not a caster (or at least student!) of some kind of magic, you're going to be looked down upon. There are different kinds of magic, where you might be seen as odd or dangerous to be a mage or [gods forfend!] a psychic, but hailed and praised as a cleric or druid, and vice versa.
Monsters: Monsters are everywhere. Again, different critters are different places. Is every land you walk into going to have a dragon sleeping on a giant treasure hoard under that mountain over there? Certainly not. Will there be a story claiming that from some centuries ago? Yeah, probably. The only "ubiquitous" creatures would be those "most common" foe types. Kobolds and goblins you are probably going to find just about anywhere. Even orcs, in my world, I've sequestered into the mountains and caves/caverns/shallow subterranean dwellers. Raids happen into areas adjacent, but you're not going to find an orc around every corner of every forest or dungeon. You're not going to find a troll (openly) stolling down a fortified town street. But there might be cautions/notices (and bounties) about one that was seen at some farmstead not far from (in fact, far too close to) the town. Want to catch and train a griffon as your mount? Better go find some royal elves or fae nobility to tell you where they get theirs (They're definitely not selling you any of their domesticated ones!). Otherwise, you can look at these pretty pictures. I heard there was one seen from the walls of Ablidon over the Whitegull Bay some years back.
Classed NPCs: Same answer. They can be common. They can be disturbingly rare. It depends where you are. You could be in a village with a middling level mage/witch/druid "healer/potion maker/apothecary." You could be in a bustling trade town with noone over a 2nd level thief in the local gang (trying to make themselves into a "guild."). You could be in a shipping hub with archmages and world renowned warlords coming and going at any given moment. Or some ridiculously high level priest who just happens to be living his best life at a secluded shrine, all on his own. He's renowned as a healer and exorcist of evil demons by the local folk, who are clearly a very superstitious lot. But no one really knows of just how [truly] powerful he is.
It just depends. Where is the particular campaign taking place (and will it stay there or move to other regions)? What are the needs of the particular party (as far as the degree of challenge to produce fun)? What are the flavor preferences or general "mood" one is going for in a particular campaign, or singular session/section of the game? A "save the princess who disappeared in the faerie wood" is going to play and contain very different degrees of all of these things than the "party must infiltrate and bring to justice the problematic bandit gang between towns."
If I want to do a "gothy/halloweeny/horror-ish genre" section to a story, that's going to involve very different monsters and magic and NPCs, than a "seek out the Lost MacGuffin of Unknown Wildland and save the kingdom from invasion." Or a few sessions (or entire campaign) of courtly intrigue and murder mystery in the Duke's castle. Or a party that just wants to seek out the fame and glory (and buckets of treasure) surviving the "Dungeon Delve of Doom."
(Personal Side Note, here, I am not sure where there are town guards who can easily take down a group of 7th level adventurers. Other than, like, an honor guard for monarchs or temple high priest with a bunch of mid-level underclerics or something like that. I can't imagine some "joe normal soldiers" or your average "town constabulary" from Commonfolkville being able to pin down -much less defeat or detain- a party of 7th level guys. But, then, that too would be a sheer matter of preferences of game/play style and flavor.)