he worst offender is handing out advantage like candy, with no resource cost. A host of abilities and spells offer the player or their team-mates advantage at some resource cost. T
That is also a BIG NO NO. There is a reason why "flanking" mechanic is OPTIONAL rule in books and why it's considered absolutely broken by majority of 5e community and almost noone ever uses this rule at their table.
Not only it makes many of class features useless: like Help Action, Fearie Fire, Greater Invisibility, Fighting Spirit, Reckless Attacks, Vow of Enmity etc. that COST RESOURCES or have a downside attached to it FOR A REASON.
Because advantage is THE thing in 5th edition. It's super powerful and it requires resources (spell slot, concentration etc.) or class features (limited to 3/day for Samurai for example or 1/short rest per 1 enemy on Vengeance Paladin) or have severe punishment attached (enemies have advantage on you when you Reckless Attack).
Handing advantage to everyone for cheap is not only stupid and destroys balance in game, but complicates classes (you have to tweak them now because you made their book features useless) - which causes more balance issues but also advantage doesn't feel significant anymore.
Sometimes I get the feeling they just want new player to feel like god from level 1, getting advantage left and right and throwing AOE firebolts everywhere that ignore AC.
This is not DnD.