When you first started designing games, you did things you thought were great, cool, and fantastic. Or forgot to do things maybe. Now that you have a lot of experience, and you look back, you know some of those things were really big mistakes. Not code mistakes, design mistakes.
So my question to you is: What mistakes did you make?
Not so much things I would do different, just things I wish I would have known before getting into it. Going back through and seeing my old code while I'll remastering everything makes me gag and ask, "Wow, why did I do it like this when it's so much easier to do this?" xD Design-wise, I guess just things I wish I could have implemented sooner like multi-buttons on the compass, for example.
In my first games, I unthinkingly copied things I remembered from '80s/'90s adventures that I didn't know had a) gone out of fashion and b) never been justified in the first place. Inventory limits, unwinnable states everywhere, and a general philosophy of annoying the player rather than entertaining them or telling a story.
Can you imagine the feedback I got on that?
As a result, I now programmed a parser entirely from scratch, guided by all the feedback. No advice was left out and it works like a charm!
A new game will be released in a few days, based on that parser and I hope you'll all love it.
Speaking of mistakes... how about releasing a game with more typos and grammar errors then there are stars in the sky, still thinking it is a great game?
I would suggest that trying to make a BIG game as your first game is where many come unstuck.
You may have big ideas of what you want to do, but mostly you have to think SMALL. Get the basic mechanics right first and gradually bring the parts together.
I have seen too many posts where someone says they are making a game that will have this, that and the other and be able to do anything imaginable. However that is their last post and the game never sees the light of day.
Big ideas in themselves are great, but realise your limitations (and that of the programming system you have chosen) and work within these and then expand when confident.