I'll answer the questions to the best of my knowledge
Stilgar Is there a reason why i wouldn't want to choose to wear heavy armor?
What are the advantages for ligh/medium armor compared to heavy?
From what I can tell you move a little slower in heavy armor as well as reduced carrying capacity die to its extra weight.
Mysteria Hm, let me add some questions of my own:
-What is the yellow bar that appears over my own bars when engaged in a fight ... I've got the feeling that it is my opponent's health, yes?
-What is this with fatigue? Aren't you supposed to pass out when it is at 0? I'm no 100% sure, but I normally run all the time and I'm don't pass out even when the bar is all empty ... strange?
-Blocking: Is there anything else that I have to do to block besides carrying around a shield?
-Enchant: Erm, how does this work?
1) correct
2) if you are in a fight and it drops to 0 then yes you do pass out.
3) no, though the positioning of your character with respect to your opponent(s) may have something to do with it as well.
4) Enchant is the ability to make your own enchanted items and being able to recharge them using filled soul gems. (as opposed to paying to have it done). it also affects how efficiently you use magic items (i.e. an amulet that uses 15 pts of it's charge w/an enchantment (your ability) of 15, will only use 1 pt. w/a enchantment of 50) not exact figures but just to give you an idea. Note that it is difficult to create powerful magic items yourself, even if your enchantment is 100 (Userunfriendly can give you some cheese methods around this

)