4. Dialogue Trees
There's a lot to be said about giving the players choice. The freedom to steer events into desired directions is a powerful feeling, and makes you feel like you're actually playing a part in the world. Games like Mass Effect do a tremendous job of this. You get to interact with characters in a meaningful way, therefore feeling more of a connection to them. Of course dialogue trees can just be fun deviations from the critical path, maybe even with humorous interludes. However, when these choices go too far in different directions, you might catch yourself regretting your choices. "Maybe I should have allied myself with the other group?" Maybe if I had told her something else she would've given me another, more useful item?" It's all fun and games (no pun intended) in theory, but there's really no way to know the consequences or benefits of your actions unless you have a strategy guide. If you don't, and you're the completionist sort, be prepared to come back to these games many, many times.