The main way is that there's a meeting you need to attend about 30 days after you leave your first dungeon. If you miss that meeting, the main plot never triggers.
You get another chance actually, if you somehow don't make the meeting you get two more weeks after another letter arrives. But you can stumble into the main quest after pissing Lady Brisienna off anyway, I know that one for sure since my first run through the game I thought I fucked up.
I think this was actually more pronounced as a problem in Morrowind however - if you didn't optimize your misc. skills at EVERY level you missed out on most of the stat upgrades, causing you to need to do dumb shit like jumping marathons to make sure you've trained the right stats just the right amount at every level. If you weren't careful in what you left as main/minor vs misc skills then you were fucked basically.
You could honestly set things up so your major and minor skills gave you what you needed in a reasonable amount. But that kind of takes understanding the system from the start... It is undeniably more important in morrowind though, given how much harsher morrowind is with the chance to hit when you swing than daggerfall. But you can still win morrowinds main quest at super early levels with an unoptimised character, it just makes melee combat painful.
Dungeons and Dragons 3.5e / Pathfinder
If by complex you mean overly build focused and convoluted sure. If any of that is actually important in a game odds are it's being ran purely on rolls and not roles. Might as well just play NWN or any other CRPG at that point.
That said I'd have to second the distant worlds mention. That game made me reconsider finding paradox games complex.