As far as the dynasties and challenges, they are great if you don't mind the rules. For me, that doesn't appeal and I enjoy the project stories. I have more control and less rules.
What makes it great is the variety for those who have different tastes, so as far as the dynasty challenges and the projects, check them all out and see what one fits for you. Or create your own personal challenge.
I did my own personal dynasty, set the number of generations, goals for my sims and didn't limit myself to set areas, rules, and used the motherlode to help with starting funds.
Ask away with questions, I have and still do constantly ask questions, try things out and go help! quite often
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As for expansion packs, there are several, and what they offer vary, there is a section that has a review of all of the expansion packs in the guide. As someone stated, go to the top where it says guide and it will take you there look on the list on the left side that has a variety of topics and you can learn a lot of what the game offers and tips on how to play the game. A GREAT help and I constantly go back to check things out when I am need some info.
I love the search engine they have here also, and the fact that I can ask follow-up questions when I don't find what I want. The people here are great to provide the help that is needed.
A brief info on what I do have on expansion packs, WA (World Adventure) is travel destinations, and you don't have a home town. Pros and cons on it, there can be some bugs on it, but I think it is worth it. SN (Supernatural) has a town, Moonlight Falls, and if you like the though of playing vampires, werewolves, faries, witches or just being a human, then it is a great one. It has a Victorian feel, with enough "dreariness" to give it that mystical supernatural look. Each expansion pack has something special about it, and some will appeal to you and some won't, it is all a matter of personal tastes. What some like others don't . I don't like Pets, but that is me.
Hope this is of some help.