Originally Posted by DArtagnan
It's hardly rocket science, though.
If someone played Deus Ex 100% "run and gun" - shooting everything and ignoring all books/conversations/secrets/etc. - I'm pretty sure he could get through it in 10-15 hours.
As for the opposite way, that would be extremely careful and stealthy - scouting all locations before making your approach, scavenging every corner for secrets, reading every single line of text in every single book/pda/hacked computer, talking to every single NPC and completing every single trivial side quest, and so on. If you're absolutely sure you did everything in every level and you STILL only played for 20 hours, then maybe you're just an extremely fast reader?
It's not about content though. In these open shooters I sometimes hang out in a hiding place for minutes, trying to assess the situation, watching guards, trying to decide on my aproach. Stuff like that can easily inflate your playtime by a factor of 2 or 3.