Been a little while since I traded my 25, but upon some perspective and reflection, I realize that there is nothing about the 25 I prefer to the 21 except the lack of a locating hole. I much prefer the bushing pivot, single tool takedown, symmetric stop bar, and have even decided I prefer the old hollow grind (longer "sharpenability" since the shoulders won't widen out as fast). If CRK doesn't change what has been shown as a 25 inlay, they will lose a demand IMHO.
I would agree with you that the stop pin is a PITA. I understand the switching to the 'D' stop pin is presumably designed to increase the surface area hence reducing stress between the blade/frame and allow 'progressive' contact to replace the traditional none-to-full contact of many stop pins...but both the mated and free-floating pins just haven't done it well. My guess is this is CRKs response to the stress generated by a metal-on-metal hard-opening and I think this is one of the biggest vulnerabilities of the titanium frame locks (as kick ass as they are)...they just havent done it before and are still working at it. Hopefully this part will see future revisions to be easier for the user. It's a good idea, but just not matured (which is unusual for CRK). With that said, the current design is more of a maintenance PITA than a functional issue. But for a CRK it should be refined better.
Same with having to use two tools...kind of a PITA and they should have just switched to an intermediate size for all. I personally hate Loctite...I use VC3 and Extreme Fluoro instead. I use VC3 on most of my knives so the need for a threadlocker isn't a big deal...I just hate Loctite.
But I find the newer high flat grind easier to sharpen. I think between the grind and thicker width it loses some of it's 'extreme slicing abilities' that the Seb has always had, but it seems to gain some edge strength. I can still use it as a paring knife and have it do fine cutting without killing soft vegs...but it def needs to be sharper than the older grind to give the same results with cutting as such. I find when cutting cardboard it holds a working edge longer and it seems less prone to rolling when cutting something like thin copper wire many times over. But that's a real trade off...my guess is some people love it and others hate it. Given S35 seems to be able to take to a 61 HRC with plenty of toughness, if Reeve just upped the hardness a little bit to around 60, the same results may have come just from that alone...he's helped make this amazing steel with the ability to improve over S30 dramatically because it can be taken to a higher hardness without chipping, but he's hesitant to crank it up. S35 and ELMAX are both great steels for a 60 HRC with toughness beyond most other stainlesses at much lower hardnesses and easy ability to sharpen...
With that said, I still really like the 25. We can be picky about it because it is a $450 knife...if it were a $100 knife, it wouldn't be fair to be as picky as we are. I think the new washer system, the ergos, the lanyard system mated with the standoff, and the new design making weak-handed opening much easier are all pluses. The thumb serrations are plain awesome and I think the best on any folding knife I have ever handled. I think the pivot/ball redesign also helps the knife do better when it is really dirty or oily.
I would have loved to see some sort of milling that is less aggressive than the Wilson Combat designs, but more than smooth. The thumb serrations and finger grooves gives really good leverage IMO, but some sort of texturing would complete it. I think that is partly why we are seeing so many titanium frame locks that use G10 or CF inlays on the non-lock side, for the additional grippiness.