For general cubing questions, visit the Speedsolving forums and their FAQ.

Why should I put the cross on the bottom (or left) instead of top?
These are the two advantages of placing the cross on bottom:
• It lets you use U and R turns, which allows you to execute algs quickly using finger tricks.
• It gives you a much better view of what you're solving and lets you look ahead.
• Is it faster to cube with the cross on the bottom or on the left/right?
There are top cubers for both, and I don't know that there's an advantage either. I do find that F2L can be done with less awkward regrips with cross on left (mainly using double layer turns Lw), but I'm not a fully left-cross cuber so I don't quite know how they really compare. It should probably be noted that there are lots of tricks for F2L that are especially nice for cross on D, but I think that's just because there are more fast cubers using cross on D.
I only know about half of the OLL algorithms. Do you think that not knowing all of the OLL algorithms is holding me back?
I don't think that it's a problem as long as you can immediately respond to every single case. Especially if you know for every pattern what algorithm or pair of algorithms to use, there shouldn't be very much difference between 2 step and 1 step OLL. I think Harris Chen was getting sub-15 with 3-step last layer, and I doubt he's the only one.
What do you think is faster? Traditional cross or extended cross?
Extended cross definately takes fewer moves, but should not be done unless you can completely see what to do.... The most important thing in cross and F2L is to not stop. After i memorized all f2l, oll, and pll, my avg was about 40 sec. And then in a month it was sub-30 and then after 2 more months, sub-20. I think you just get used to it gradually. In the meantime, there's not much you can do but to practice.
I can do F2L intuitively all right, should I still learn the algs?
Once your time plateaus, it might be a good idea to go through the algs. Learning the algorithms shouldn't be very difficult if you can already do it intuitively, but it will allow you to look ahead more easily).
How long did it take you to finally memorize Fridrich's method and how? Do you memorize the notations of the cube first?
It took me 2 months to master two step F2L and OLL and average sub-40, 3 more months to memorize full Fridrich and sub-30, and another 3 more months to be able to "see ahead" and use finger tricks and finally average sub-20. Yes, I did memorize the notation first. After starting to learn Fridrich, it took me about a year to get down to 20 secand another year to dip below 15. However, I wasted a lot of time by first learning F2L with cross on top and also by spending time optimizing last layer algorithms. With good algorithms and practice techniques, you can get sub-20 much faster than me.
How do I get REALLY fast? How important are the advanced F2L tricks?
You can get to sub-12 with just an amazing lookahead. Beyond that, you'd need to get a lighter cube and start turning faster. Today's top cubers regularly hit 7+ tps. Knowing advanced F2L tricks can only help, though I'm not really qualified to say how much. Multislotting is less important.