These are two very different games, so I wouldn't necessarily expect things like the
rarity or mana value to reflect their D&D counterparts. PW Kill is just an iconic kill spell, so mission accomplished. Agree about Tasha's, though, it's just a lame mill-esq spell that has nothing to do with the original. Perhaps it's hideous because milling out is such a hideous rule, and the devs are laughing at us because it's so terrible? :-) Sygergy with the cool Ashiod, though, I guess.
Tiamat actually has a nice, if kinda boring, play pattern as a commander. Just pack mostly ramp and a few game-ending dragon bombs, ramp up to her then her ETB tutors for a handful of stuff you can also now afford to play. Not exactly competitive but it feels like a fairly easy deck to build, and functional.
I played against a dungeons EDH deck the other day. It wasn't super powerful but was quite cool, and I don't think the guy had exactly tweaked it yet.
On the whole I like this set, though the lack of Party is a massive bummer. They finally go and print a good mechanic, but then we have a perfect
opportunity to see more of it, and what do we get? Dice rolls? They really missed a trick here.