What's your least favorite parts of previous Igavania games?

This has got to be my biggest gripe. It's bad enough having to wait until nearly the end of the game before you get the ability to move at a reasonable speed, but not being able to keep it makes new game plus feel like such a slog. You get a taste of speed, then are brought back down to a crawl. I feel that it really distorts how I play through them -- I end up racing through as quickly as possible to get the speed ability, rather than enjoying the second playthrough. Whether the ability limits progression or not doesn't even need to be a consideration -- you've already seen the story, what's it going to hurt if you see events or get items or abilities out of order? New game plus should be there to allow you to enjoy what you've acquired, and work on completing what you haven't. If you want to start from scratch, the option's there.

My second gripe would be mindless grinding. It's interesting to devise a strategy, find the best room to grind a particular enemy, the right equipment, etc. but actually implementing that strategy, ducking in and out of the same room one-shotting an enemy over and over for hours, either for experience or a rare item, is incredibly boring and annoying. I had an epiphany grinding EXP in OoE, ducking in and out of the Double Hammer room in the Training Grounds, two-shotting both of them (does that make it the Quad Hammer room?) with Acerbatus: how is Shanoa learning anything from this? How is repeatedly blowing up a couple walking suits of armor making her stronger? It shattered the illusion of experience representing the character learning and developing from combat for me -- instead, it was just some magical resource that enemies drop when they die.

I like that randomness makes each playthrough a little different, like using whatever drops, playing around with different weapons and abilities, rather than grinding for the best thing, but once you've gone through the game, and are just trying to complete your collection, there should be a way to speed up the process significantly. No, the luck/rare/soul rings aren't sufficient. There's been plenty of debate over whether they do anything statistically significant in some of the Castlevania games, particularly DoS. I actually liked the stealing system in Curse of Darkness, though it certainly had its flaws and could be frustrating. It had that same element of hunting down the enemy and devising a strategy, but substituted skill for most of the randomness. I'd like it if using the right ability, or the right timing, or something the player could discover and choose to do, would drastically increase the chance of a rare item dropping, if not make it drop outright. Stealing glyphs in OoE is another example, though it wasn't used for very many of them.