Some of my favorite decks include many different debuffs (Stuns, Hexes, Blinds, Soothing Serenades, etc...) When a deck has a lot of these it can be fun to play, even if the champions with them tend to have low damage. I also like it when theme decks are fleshed out with playable theme-specific champions, spells, relics, and equips.
Units with the same race /class boosting each other to work like army not just slaping in random 30 runes in deck. Also Things that have Drawbacks like mentioned foul rite
I really like backfire spells like lifeshield, snowblind, backfire boon and I think this brings great moments of counterplay. Abilities that grant backfire like counterspell mastery or spell blockade are either badly designed or overcosted. Counterspell mastery would be greater if you could chose whether you want to use the backfire or not when the champion is fully stacked. Also i like things that makes your things more efficient, like leverage beast, camaraderie, because It makes you play simple stuff but with efficiency instead of meta champions. Also I like spellsurge, council spell and amplifications because it makes your whole deck synergise.
Also, things you can dispel, sever, shatter, distract, stun and defile and any other counterplay mechanic.
Since playing de'lim i can tell you, i really like the ones where you have certain champions that are expendable resources that go towards buffing your others, Thats also why i enjoy the vengeful BGs from FW. So more on this stuff is excellent thanks!
I like Ironfst Exploit and Truthseeker. Although I would like Truthseeker more if Faithless were changed for something else
I dislike swarms, summons, excessive AP gen, nora gen, runes based on rune usage(such as equips or spells), LoL, or any mechanic that triggers too much in a given situation. I kinda like the rest.
I like more realistic abilities and design. For example, why can a small champ throw a heavy shield five spaces instead of maybe 2? Why are centuars, half man and horse, as big as dragons, titans, etc.? I like better PVE options. I like everything on your list, the more complex the better.
I like debuff BGs. Sand, witches, slags, etc. things which decrease you opponants efficiency more than increasing your own