Well, my problem with Castleroids in general is like this - they all for the most part, felt the same to me.
But--feelings must be controlled! =3
No matter how much sugar-coating and new subsystems the game had, in the end I felt like I playing simple addon to the SOTN, not a separate game. Only two games came close to be considered as more or less separate entities - COTM and OOE, that actually did more than just having trying to retread SOTN for N-th time.
Generally I just don't get that feel, which is probably why I'm not tired of them. I can perhaps try to expound on why but it's difficult for other people to understand why games feel a certain way to other people.
CotM: A whip as the only main weapon makes it a bit different than SotN, DSS system was actually somewhat different for its time compared to anything SotN did since its spells generally faded into the background and the traditional subweapons with their traditional hitboxes were the main form of attack besides the weapon. The castle layout definitely felt different, if worse due to all the obvious generic obstructions at random points in rooms dedicated just for them (the box rooms and the slide-required rooms). Enemy set was pretty different if unfortunately having a lot of palette swaps. Besides the central hub area, the areas felt more "blocky" in layout than SotN as well with their own dedicated spaces in the map. The graphical style was probably closer to SotN than either HoD or AoS, but not exactly a match.
HoD: Another whip game, and the weird atmosphere the graphics and the 8-bit music generated pretty much dominated this one for me, making it hard to get the same feel as SotN, despite the genre conventions in use. The Subweapons combining with the spell books to produce unique spells felt more like a take on CotM than anything out of SotN really. I suppose the subweapons should make it feel more like SotN but ended up giving me more of a classic feel (probably due to the worse graphics as well), so that sort of deflected that possible SotN feel being there for me. Also, the lower graphical capabilities of the GBA meant that they didn't reuse SotN enemies' sprites yet, so that also made it feel a lot less connected to SotN. The second castle and the 'collect Drac's parts' sidequest for the good ending were about the only things I recall off the top of my head that were reminiscent besides the formula-specific stuff like equipment and items to open new areas.
AoS: In gameplay this one feels closer to SotN than CotM or HoD due to the return of nonwhip main weapons, but the graphical style, not as weird as HoD's but not as high quality as SotN's still doesn't remind me that much of SotN. The colors are still lighter and it generates a different atmosphere for me. The soul system feels more like a change up of the CotM-HoD spell system than anything in SotN. The plot tends to differentiate it given that it's in the future and after Dracula's demise (perhaps an equivalent 'change' to the oddness of Bloodlines taking place all over Europe, unless one really puts a lot of emphasis on the plot).
DoS: Now due to the return of the high quality graphical style, visuals feel more like SotN than any of the GBA games, but the snowy entranceway still seemed to set up a different first impression of the atmosphere that I can't shake and some of the areas are odd change-ups atmosphere-wise compared to SotN, like the garden area or the silenced ruins. The caves and clock tower have more of a SotN atmosphere. Actually I don't think any of the castles have given me the exact same atmosphere though, besides perhaps DoS and PoR. Soul system is rehashed so again that element and the returning Soma made it feel more like AoS. There's a bit of an unwanted innovation of souls powering new weapon creation but it isn't too huge a change.
PoR and OoE definitely started feeling even more alien (compared to the SotN formula) to me given the layouts being more linear. The PoR castle of course was reminiscent of SotN but it did not end up really being the main focus of the game, and there was a reduction in the "collecting items to open new areas" count due to the more linear, self contained, and totally "open" portraits taking up a lot more of the game than the castle. By "open" I mean that once you got into one you knew you likely had everything you'd need to go through most of the portrait world and reach the boss at the end rather than having to frequently backtrack to the castle or another portrait. This of course was a precursor to OoE doing away with a castle "hub" entirely and going to the world map system. Spells as weapons also felt a lot different because of all the weird hitboxes you could use as a "regular" weapon rather than some rare-use side attack. The combat requiring a lot of alternating Y and X often made it feel a bit different than the combat in the previous games. Like with PoR, the castle once you get to it is generally too small to provide a lot of "collect item, use it to progress through some past barrier" issues. It was still there occasionally but reduced and not really the focus it was in CotM-DoS.
So in the end, for me, it ends up being more like: There are games which feel like SotN in noncombat gameplay (CotM-DoS), games which feel like SotN in combat gameplay (AoS-PoR), or games which feel somewhat like SotN in graphical style (DoS-OoE). But far from having every game possessing the exact same elements there together, some of them have some aspects of SotN's gameplay or graphical style, and others offer a bit of a change up. The RPG system, however, is just so innate that I expect it to be there rather than wanting it to be changed up.
After typing that, it seems to me you focus more on the core mechanics of the games and that's generally why you get the same feel, the RPG (level, equipment) system, the "formula" of finding new items to access additional areas (although PoR and OoE didn't have as much of this given they have other methods of restricting your egress), whereas what I end up reminded of in each game is differing atmospheres, castle layouts or spell/attack mechanics.
I find that generally when I don't much like a genre a series is using at the time I end up wanting more innovation in the formula. For example, I'm generally lukewarm on Zelda after ALTTP and I'm always waiting for them to change up the dungeon-item-boss-collectable formula, but I notice the majority of Zelda players always clamor for the next Zelda and don't really care if that formula changes. You may feel similarly with the castleroid/metroidvania formula.
I am not sure, maybe its just only my personal feelings about the matter, but I think that's how some other people feel about "metroidvanias".
I've seen other people express similar views. Of course I've also seen other people begging for another metroidvania/castleroid as well or entirely expecting that one will come. I expect it'd probably still sell at least on the level of the DS CVs if they were to make another.
The bad thing that dual partner system in POR was useless for the most time. It was only useful in the begining of the game, when developers thought of providing some puzzles to solve with this system. Later they mostly forgot about it and the game was comletely beatable with only one character, except for the few boss battles.
I think it could have been a major innovation, if it was done right. As it was executed it was more of a gimmick for the begining of the game. Like only 20% of what it could have been.
These little puzzles are actually a bit more spread out than you may remember. For example, the train puzzle was in the final "13th street" portrait before you gain access to the series of final boss fights, the go-kart thing is around the half-way point and some of the simpler implementations are earlier in order to access some of the early portraits. Surely though, these puzzles didn't pervade the game or anything, but I think that was probably intentional so as not to interfere with what the player was likely mainly looking for, which was less of a puzzle platformer and more of a combat-platformer, with the puzzles as more of a side thing.