DS1 was just a rare case of a well-polished popamole game. Nothing more, nothing less. Arguably, I'd say it's a contender for 'best game of the shithouse dumbed down popamole genre'. Yes, that's damning with faint praise, but they took what they could do with an EA game, and made something remotely playable that bent a few expectations - switching the focus away from headshots was original at the time, the return of location-specific damage in a way that really affected gameplay and could be used tactically (taking out the legs to slow a powerful monster down, or taking out the arms to limit its attack) and sometimes HAD to be used tactically (the regenerator). The plot was decent-to-good for a game where plot wasn't really the point, and it was brave enough to break a few rules (allowing you to die quite easily in the tutorial area if you don't gtfo when told - which was also quite a clever warmup to later sections of the game where you had to turn tail and run - not that 'having to run and dodge enemies rather than fight' is original per se, but I don't recall seeing it in a popamole shooter before.
In fact, if the makers of DS1 had immediately broken away from EA and decided to make their own game, I'd have been genuinely excited to see what they could do - they did a good job of bending EA's notoriously strict rules, and they bent them further than many of the 'great developer-studios' that EA has purchased, so without claiming that DS1 was anything but a mediocre dumb-fun game, they more than earnt the chance to show what they could do without EA leaning over their shoulder.
Sadly, the 2nd two games just turned into 'milk the franchise'. The real nail in the coffin for me was that they could come up with great set-pieces like the re-generator level and some of the 'holy shit kill that squid thing before it turns that corpse into a....oh fuck.....' sequences, and did a great job on harder difficulties of keeping the ammo at a point where you really couldn't afford to just blast away without utilising the location-specific damage mechanic, and yet in DS2 and 3 all they did was copy those same ideas with worse implementation. It stank of corporate miliking - don't bother giving the guys the chance to come up with something as fresh and interesting as the regenerator level (if it's even the same guys working on it) - just tell them to use the same thing, except more often and with more obvious solutions so it kills any novelty it every had.
I'm not saying that DS1 was a masterpiece by any standards. It was a decent dumb fun game, that's it. But in terms of developer achievement, it was also something of a triumph against adversity - a game where the developers seemed to sneak in the occasional bit of originality and challenge under EA's noses, in a way that makes you suspect that they were trying to push the boundaries of a popamole game without EA noticing. I've often said that Deus Ex should have become 'the standard' for games in the fps/rpg hybrid market - the mechanics are simpler than any of the Bethesa games, and anyone who can play ME or FO3 should be able to easily pick up the original Deus Ex without difficulty (maybe with a more in-depth tutorial and updated graphics, but the mechanics themselves are no more complex than ME, and considerably less complex than FO3). In a similar way, DS1 should have been the standard for 'games for retards' - and I mean that in the nicest possible way. If you wanted a game with minimal thought and ultra-simple mechanics, but executed and balanced well, with polish and as much creativity as the popamole genre allows, then who shouldn't the game be of the standard of DS1, rather than its sequels or the godawfulness of the CoD clones? Hell, there's certainly times when I want to just switch my brain off and play something ultra simple - if I have insomnia I don't go play JA2, I play CS because I'm in no state for actual thinking, and just want to kill time, so there's a genuine place for games like that. It's a pity that when a game comes along and very marginally increases the standard of that genre - even without making it any more complex - the corporate overlords immediately stomp on anything original or challenging to have come out of it.