Strange things about 5e:
* Almost every d20 roll is one of three things. An attack, an ability check (skill rolls are ability checks, as is initiative) or a saving throw. The three act very similar, but different rules apply, mainly in what modifies them.
For example, bless boosts attacks abd saving throws. Guildance and Jack of all Trades boosts abilith checks.
* When a PC wants to do something and it isn't an attack, ask for an ability check. Pick an ability. They can then ask "will religion skill help"? If so, they also get proficiency.
There is a chart in the for what the DC should be; "basically impossible" is 30, for example.
If someone is trying to break a door, the check is not for bashing it once, it is for trying as hard as you can. You don't automatically get a new check if you say "I bash it again"; you could lower the DC if you find a better solution than your boot.
* CR math is annoying. There are encounter size multipliers are meh.
But the 5e power curve gets pretty flat from level 5 to 10.
So eyeballing it works if you dislike math. (I like math, so do complex naughty word to fix it; many alternative systems exist).
Large numbers of foes can slaughter PCs faster than big dragons.
* The short/long rest structure is the hardest thing to keep on an even keel if you don't want certain classes to make others look like incompetents.
If you are running stuff yourself, consider the "gritty" rest, where a short rest is overnight and a long is a week.in civilization. It makes a lot of non-dunegon crawl plots work better (but it does weaken PCs significantly; so module plots won't work as well).
* Flanking, while cool, ends up making advantage less cool.
Give out and take advantage liberally in combat (to monsters; have them claim higher ground, for example). Don't do flanking, because it takes away that minigame and replaces it with a boring one.
* Determine how much you will permit rogues to hide in combat. If it isn't often, let a rogue PC know and possibly make a different PC (a gloomstalker ranger makes a decent rogue that doesn't rely as much on in combat stealth, for example).
* UA is playtest stuff, often has issues. Setting book stuff is a notch above. The "everything" books have, so far (sample of 1) been more solid.
As others have mentioned, the PHB ranger has issues. At level 1 they are a weaker fighter with two ribbon (mechanically weak) abilities. There is lots of UA and Xanthar's material for Rangers as a result.
* My biggest problem with feats is that monks lack many fun feats to pick that provide hefty boosts.
* Most magic items require attunement; this puts a 3 item cap on what each PC owns. 5e is designed so that there should be no magic item shops; sure, items for sale, but you should treat items for sale like they are a treasure horde, not a buffet.
