I think it was too slow for the majority of players today. It had elements of all the game genres and until about 7/8 of the way through you could still be killed pretty easily. I thought the hardest way to play it was using the walking-tank/killing machine method.
Your character started off sucking at everything and it took a lot of time to build up the skills on any single weapon, there was no BFG you could find and just run-n-gun through the maps, ammo was always a problem, and if you didn't explore each map pretty thoroughly you missed too many upgrades/sub-stories/characters/plot lines. It was just obviously a work of love for the genre.
There is no corporate model today that would allow for the development time it took. One of the big faults/strengths about is was that you pretty much had to decide how you were going to play it in the first scene at the docks because weapons and upgrade choices you made from there were not cross-functional. If you tried to change your mind too many times or too far in, you could end up building a character that just wasn't good enough at anything to finish the game. A feature not particularly conducive to 12 year-old boys that just want to shoot everything, moving or not.
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