Would Pillars have anything you'd even consider a strength?
Sure, as I said before the original Pillars release, I think the main strength of PoE1 is probably character *building* (in other words, designing your character) - there are lots of race, class, skill, talent and item combinations and that seems to appeal to people who are into that part of playing games. All of these combinations work to an extent that you could finish the game with them on any of the standard difficulties. You can design your character, play it (and it will work) and win (provided you pick up the combat). This was a design goal that was achieved quite well, not without compromise of course.
The Codexers who seem to have enjoyed the game enough to continuously post about playing the game in the Pillars threads seem to be people that enjoy character building/designing their character.
For a lot of RPG fans - this is enough to keep them interested, but for the general gamer as a whole, I don't think it is. For me - it's a set and forget part of the game unless the game requires higher interactivity with your build to progress - such as AoD.
The reason why I think people didn't return for the sequel is mostly because of two issues combined - I don't think people enjoyed the combat, and I don't think the story was good enough to keep people going. Set aside all of the specific issues that I may have had - engagement etc. I think that the combat was just different enough from the standard feel of top down RT(wP) combat to throw people off / make people feel uncomfortable. The game is a bit fast and a bit lethal (the first 5 seconds in combat have A LOT more weight than most similar games) and requires getting used to. I think enough people couldn't figure this out that and died or didn't like it enough, and stop playing out of frustration.
The Temple of Eothas in the first town was also probably a bit on the hard side (great for us) and I think that area combined with the combat style frustrated the hell out of many first time players and I think the a large portion of the people that didn't complete Act 1 were done in because of these reasons. It's kind of funny because it's an optional area, but I think most players who got to that part of the game entered that area and didn't realize that it wasn't compulsory.
I think the story is responsible for people dropping off before the end of the second act. When the main story falls, it then leans on the side content to keep it up. I don't think that the side content was at a good enough level to carry the midgame, actually that's where the side content is the weakest.
So yeah I think that the first game is definitely a contributing factor to the low sales of the sequel.