Welcome back!
Combat. So far, it runs nice and smooth! Without any super complicated stuff, or heavy tactical elements that almost require the use of mini's to keep it all strait. (that was one of 3rd ed's main flaws, imo. Attempting to marry the pure TTRPG with a miniatures game in one ruleset. Lots of feats and special abilities made combat a pita at times.)
Advantage. An effective way to deal with the myriad of things going on.
The last 2 editions of the game pushed it to become very mechanics heavy, which made combat very long and everything very complicated. In 5E they tried to keep the concepts that worked from those editions, but return to the more streamlined play of AD&D.
Cantrips. I really like that cantrips are unlimited now- and have incorporated the basics every caster would know and use often, as well as some decent attacks. Few things were more irritating for mages etal, than running out of their limited spells per day- and thus having little to contribute to subsequent encounters.
Scalable spells. Essentially, some metamagic feats just got rolled into the default magic system. Like this a lot, too. Want to get more out of a lower level spell with potential? Use a higher spell slot to cast it! Simple and elegant. They also kept the old system of certain spells improving with caster level. The whole makes for a pretty flexible way to handle spellcasting.
The magic system is much simpler and smoother than prior editions, allowing casters to be useful without spending resources.
Advantage. The rules and conditions for this are scattered throughout the books. I didn't even notice it- until I drew up a Rogue, and went to use his Sneak Attack. There's essentually no guidance included on how the Rogue can GET the advantage needed to use it. (without spells and stuff)
Advantage occurs whenever the DM decides. For a rogue the primary method is by becoming Hidden using the stealth skill.
Character creation and advancement. The downside to all the options, is that it takes longer to draw up a character- or to level one up- as the relevant info is spread across several chapters, and you need to look in several places to make sure you have it all. Background, race, class, domains, archetypes, school- all have a part of the pie, and most advance with your level.
Magic schools. Okay this is a niggle, but I really wish that since choosing a school of magic is now part of the core for every Wizard- that there was a list of spells sorted by school!!! Or even an abbreviation tacked onto each spell on the main spell lists. Considering there are class abilities tied to your use of spells of a specific school... Cross referencing each spell individually is a pain in the tookus!!
There are a lot of online tools people have made that can help with this. Unfortunately the original books are not the best organized, and the index has been considered a joke for quite some time.
OUCH. Some monsters are surprisingly tough in play. For no apparent reason. My party was VERY nearly TPK'd by a pair of giant spiders! At the end of that encounter, the mage was unconscious, paralyzed, and at death's door.
The game uses the CR metric to determine how difficult a creature is, but every method they've come up with since 3E has had flaws. Shadows are a CR 1/2 that are still a problem with 10th level characters. The giant centipede is a CR 1/4 that can kill a 1st level character in a single, non-critical hit (1d4+2 and potentially 3d6 poison damage).
- some of the classes got totally rearranged/redone, and I don't like the way they're presented. Sorcerer and Paladin, for example. I think the Sorcerer's fluffy bits are too specific, and limiting. (ALL are descended from dragons now) And not a fan of incorporating wild magic (a campaign specific phenominon) into the class by default. Imo, this likely came as a result of the new magic system completely blurring the mechanical differences between a Sorcerer and a Wizard. The Paladin, in contrast, is too general.
The theme of a lot of classes has changed over the decades, paladin most of all. Rather than being a holy warrior of Lawful Good deities, you're now a devoted servant to a sacred oath. Sorcerers are in some way inherently tied to magic based on their sub-class. The dragon sorcerer is descended from dragons, for example, but a wild mage isn't. There are more sub-classes in expansion books that give other options as well. As for wild-magic, that wasn't a setting specific thing, although it was introduced in Forgotten Realms (the wild mage was originally printed in a generic 2E book). Unfortunately since 2E aspects of the Realms has slowly bled into the basic D&D rules; check out the height of elves for example.
Even when we do 6-8 encounters per adventuring day, the game still isn't lethal unless you do hard and/or deadly encounters. YMMV of course and if it works to give you the game style/ difficulty you want, kudos for you.
The encounter building assumes that no one is really in danger of death unless you're in a Deadly encounter, so that's working as designed. 5E is "easy mode" by default, but it's not that hard to ramp it up a bit to make it lethal. IME the lethality is limited after level 5 because of resurrection magic like Revivify. In my first campaign from level 1-18 had about a dozen deaths above level 5, but the only permanent one was against the cleric.