Stay away from Giants...
(At lower levels anyway)
Not necessarily? They won't attack you as long as you don't mess around like a bull in a china shop. They warn you adequately when you come too close, idem for the mammoths.
6-Some quest can fail if you take too much time to complete them
Eh? Wait, what? Seriously? They do warn you in time about that, do they? Pretty please? *sweat*
As far as I've heard, some minor quests can fail if it concerns exploring in a cave.
Stay out of trouble Elf ...(if you're an elf)

Anything particular when you play with an elf race (especially a high elf)? I'm planning to play with a high elf specialized in illusion and destruction next. ^^ (I know about the imbalanced destruction school, so no need to tell me about that)
enchant everything you sell
I'd say money gets pretty easy to you, not really needed to get even more. Unless you want to play godlike of course. Plus, the most interesting armor and weapons can't be bought.
Unless your a mage stay away from vamps
unless you're in a draugr dungeon with lots of delicious traps. Took a while, but the vamp master had no choice but to succumb, heh heh heh.
The Companions are disappointing.
Something kinda related: http://www.cad-comic.com/cad/20111123 xD
I'm going to have to be blunt and say I'm very disappointed in Bethesda's decision to use Steam to distribute and required to play it. Although I've been given a copy and tried to play it, I am so unhappy with the whole setup I am probably never going to touch the game again. And unfortunately the key is permanently attached to me, so I can't even give it away or return it. Shall I just burn the disc now, or start using it as a coaster?
Plus requiring steam, if for some reason you don't have internet access (and I may not on a machine I would want to play it on), you can't play it at all. Poor decision. Plus I HATE DRM!
Huh? Steam only requires internet to 1) activate your product, and 2) update it. You can easily play it offline, if you're willing to check the settings for a bit. I already played it offline, so it does work. The only sad thing I've noticed about it is that you can't add your own music to the bgm anymore, like you could in Morrowind and Oblivion. I'd love to hear some Two Steps From Hell in the background, especially "United we stand, divided we fall" while a dragon attacks. I've been looking forward to that for so long. ;_;
So true. Evaliir Srinna of the College of Winterhold is curous about the Companions but hasn't spoken to the leader or joined them. She's never joined a Thieves Guild or the DB either. She wonders what they do but isn't curious enough to join because it's just not her way.
So this means that after 250 hours of play and not even being close to being done, there are 3 other parts of the game she'll never even see. I wonder how many more hours those encompass? Wow...what a huge freaking game.

Yeah, my character won't join the brotherhood either, 'cuz she's just not that type. I'll leave that to the high elf spy I'll be creating next. That one will be designed to create havoc and chaos, so the civil war at Skyrim will be on its peak when the Thalmor come to conquer it, heh heh heh. Iizlovaas (my character now) isn't even using imperial weapons, 'cuz that's against her principles. She prefers ancient nord weapons.
On-topic with the thread:
Erase the map marker (by clicking the respective quests off. More searching, using you sense of logic, and puzzle solving!).
Take your sweet time.
Don't stick to roads.
Npc's dislike Unrelenting Force (had to appoint it to a different key, because I accidentally hit it too many times)
Keep distance from partners (unless you want to kill them)
Better yet: Keep partners at home (in the kitchen if you can)
Sabre cats aren't pets.
In battle, Use your surroundings (i.e. rocks, or a giant camp, etc.) (blah, I sounded like an orc when I tried to make it a four word sentence

).
Markath is a frigging labyrinth. O_o
Guards give valuable information. (Sometimes. And I'm not talking about the arrow to the knee etc., I'm talking about their warnings of certain places. Stay put and listen, and your map will be updated.)