This thread made me go play it today. I played it today on Hard Mode (with full, modern jumping controls) and 3 Lives. That's the way I first played it, not originally knowing how much the other modes changed the game's content and wanting a decent challenge after paying $10 for 6 levels...
Well, let me tell you, it's a really good "Classicvania" experience. It's been a while since I played this game's stages linearly, and I spent an hour-and-a-half, and only got up to the midpoint of Stage 4. For as simple as it may seem, THAT is how challenging and deep this game can be. Most of that time was spent struggling through Stage 3, but there is a real sense of reward, and you cannot truly see everything in one playthough because of the alternate routes within levels--some hidden behind walls, some locked by keys or other requirements.
Replaying it, it's got a little bit of everything Castlevania, pre-SotN. I would say it's mostly a mix of Rondo, Bloodlines, SNES Dracula X, and Castlevania Chronicles (Sharp X68000 version, non-arranged). I think the challenge on hard is maybe most like that Sharp X68000 game, actually. Even though it can be frustrating, I had a ton of fun playing it again. It hits the core of Castlevania extremely well, and actually has a fair amount of personality and unique identity within its mixture of game designs. It has some nice, obscure homages to Castlevania throughout, and the bosses (and mid-bosses!) feel fresher than usual. While most of it is breakneck action-platforming, there are moments where the stages are built around puzzles similar to, say, the MSX's Vampire Killer. Some reviews state that it has a bland aesthetic, but despite being slightly more cartoony/comic book-like in tone than I'd like, I find its stages and set pieces exude a pretty strong atmosphere with a good sense of consistency/continuity. There is an especially unique platforming sequence in Stage 2 that I love, and one of the alternate routes/short cuts teases a potential mid-boss from a later level.
Rebirth is probably still one of my favorite games from the last seven years. I had hoped it would mean a more embellished sequel that would mix Belmont's Revenge with Super Castlevania IV...but alas... As much as there is to be happy about the recent Igavania news, I don't think it will scratch the same kind of itch that The Adventure Rebirth does. There's a driving purity to its design that the Metroidvanias have never hit.
Here's an important tip: In this game, the whip's "hit box" extends slightly above and behind your character. This is crucial in tight spots, letting you go on the offensive and defensive all at once if you time your whip swings right.