Like:
Music
Difficulty(most of the time)
Gameplay
Characters
Dislike:
fanbase
Difficulty(some of the time)
most fangames
All the gimmicks
As for examples of difficulty that I like:(PC98 not included in any of these, but there's plenty of them for each of them)
Virtue of Wind God(when it's not throwing solid walls), Kappa's Pororoca/Illusionary Waterfall, Illusionary Dominance(MoF and DS), Agni Shine High Level, Rage Trilithon High Level, Water Elf, Mercury Poison, Scarlet Gensokyo(Hard), Everything from Kaguya on IN, Orin in general, Most of UFO on Normal and Extra, Murasa last normal spellcard, Grudge Bow, QED, Descent of Izuna Gongen, Boundary of Life and Death, Mishaguji-sama(although this one is easy)
Examples on difficulty I don't mind too much either way: Patchouli books(these seem fairer each time I go against them, but I still can't capture them. The BS is in clipdeath, not actual reading). Demon of Purification(I'm just terrible at these ones though, but they're still fair), Illusion Dream Vine, Colorful Chaos Dance
Examples of Difficulty I hate: Peerless Wind God, Sylphae Horn High Level, Agni Radiance, Scarlet Meister, Remilia Opener, Eternal Meek, Young Demon Lord, Scarlet Gensokyo(Lunatic), Princess Undine/Bury in Lake, Killing Doll, Ichirin on Hard/Lunatic, Shou first nonspell, Shou third nonspell, Byakuren on higher difficulties, VoWG(when throwing solid walls), Almost everything in PCB, Eirin on IN, Marisa's nonspells in IN, Kogasa(Hard/Lunatic), UFO Hard/Lunatic in general, Yamame's opener, Heart danmaku, Stuff that's impossible unless you memorize it, StB/DS puzzle gimmick cards that are impossible to clear with pure dodging, Vampire Illusion, Momizi, PostMomizi BS Walls, Kanako opener, Stage 5 midboss Orin.