If people would not run around and collect quest in bunches, instead of completing them as they get them - one for one, then they would not be stuck with broken quests. I have completed all quests! The ones that I couldn't (two in fact) had a glitch.
Since quests are being added all the time (Jeraphon somewhere mentioned there are almost 200 now), you can only know that you did all quests when you have a list of them, which is only available to the devs. For a dev, it is easy to do the quests without conflicts, so your vote does not count

. EIther that or you are talking nonsens. FOr all other normal players, here are some eamples/reasons you can get conflicts, without it being your fault:
- you talked to a menki about work and got a quest without asking for it (at least that used to happen). This quest was also bugged later on
- you talked to a guard, said 'yes' to some question, but went off searching for what he needed without waiting a minute for his answer, so that quest step did not register.
- in the middle of a conversation with an NPC that did not understand anything you said, you logged of. Next time, he appears to have forgotten about you.
- you have quests stuck due similar things as those, do not want to stop questing and do more.
- you start a quest that requires items that are difficult for you to get. Instead of powerleveling for a few months so you can get them, you decided to do some other quests. Which might have gotten stuck because an NPC refused to talk to you about any quests other than the one you had with them.
Hopefully all of the examples I mentioned are gone, so you can just mix up quests as you like.