1. Main Mission length
I had a few people play the game lately, and (of course) the estimated game length varies according to your playstyle, but if you strictly follow the main story, and do no side quests or anything else, it should be roughly 6-7 hours.
2. Main Mission + Side Missions length
Like the previous question, it will vary, but if you're going to do the main story and all of the side quests, it should be roughly 14-15 hours.
3. Do choices affect the game world?
Absolutely. The choices you make will not only affect the narrative, it will also change how people perceive your character. If you make a choice to affiliate yourself with a specific faction, and one of your companions doesn't like that decision, they will make that known to you, and they will decide to leave you. If you play as a character who decides to deal with every problem by killing, the world will react to that. Some people may not like the way you approach things, and will be reluctant to give you missions. If you're successfully helping a faction out, you may see a greater number of them patrolling, etc. Those are some of the ways that the choices you make affect the game world. There is a reputation system in the game, and the world reacts based on the reputation you have with each group.
4. Replay Value
One thing I really want to make sure that this game has, is replay value. You will eventually choose between a few different factions in the game, and each faction will take you through a different journey, and give you a different perspective of the game world. So each playthrough will be different depending on what faction you choose, and how you decide to approach each mission. Also, some of the choices available to you will depend on how you distribute and upgrade your characters attributes and skills. So, even if you choose the same faction on your 2nd playthrough, you can play as a different character who has different attributes/skills/perks, which would enable you to approach things in a different way.
5. Dialogue trees/skills
Dialogue trees are set up like they are in many RPGs. You will always have a wide selection of choices available to you when you're confronting a problem, and sometimes, you will have an even greater number of choices depending on your characters attributes, and skills. You can lie if your lie skill is high enough, or use your charisma to persuade the person to do something you want them to do, etc.
Skills are restricted based on what your characters attributes are. So if your Charisma is too low, there will be a cap on how high you can put your Lie skill, or if your Intelligence is high, you will be able to put a good amount of points into your Hacking skill, etc. Also, how you can approach things, and the routes available to you will change based on your skills. Example: you can hack into a terminal if your hack skill is high enough, or persuade someone to do something if your Persuade skill is high enough, etc.
