Well, first quest i received from noticeboard, upon reaching Velen, a contract about the missing brother, level 33... I went to do it at level 5. Found everything, looted a bunch, died from the boss in one hit... Thing sucks! I mean, in the witcher 2, you could craft the dark armor sets and venture out to make short work of anything moving, flying or hiding behind that ugly forest booth; level irrelevant and regardless. Here, you have to level up A LOT! And probably, postpone things.

I am aware that the most experience is yielded out when you follow the main storyline. I am also aware that following the main storyline too constantly and religiously, might cause some secondary quests be locked out and lost to you. In Velen,

1)HOW MUCH can i actually proceed to level up and do the main story, until i can safely tackle ALL secondary quests? The levels are crazy (quest requirements)! And i don't want to loose on ANYTHING, damn!

Also, i found out some monster nests actually respawn. And when you clean them, they yield almost 100 xp, a huge sum (for the beginning, since 11 nests can level you up).

2) Is it safe to use them for farming? I mean, do bombs require alcohol to restock too, much like all alchemy stuff carried?

1. You generally cannot do levels until they are green, i.e. within 6 levels of you. I wouldn't even bother trying until they are green, if I were you. Monsters will get enhanced HP, armor and damage until you are green with them.

2. Bombs require alcohol too. However, there is no end of alcohol. I probably had over 200 alcohol when I finished. Pro tip though, do not farm (for exp at least). It is pointless by design because exp for monsters is so low. Exp is through quests, not monster killing. Feel free to farm if you need some certain monster parts (like nekker hearts or arachas vencom, both relatively rare).

Don't do quests higher than ~2 lvls from you. It's quite simple. Stick to clearing the quests that are within your level range to not die and maximize XP gains and you will inevitably have some quests greyed out if you're going for 100%ing. That contract quest is really close to end game, so just ignore it. You'll be in kaer morhen before you're able to tackle that quest.

I tried to do it on launch week my self on Death March. It's do able but you'll have to have a lot of patience and ~15-20 mins of flawless not getting hit combat. Made it past half way his HP but that was about it, gave up an hour or so after trying.

There aren't that many quests you can lock yourself out from by following the storyline.

And XP isn't an issue. You won't be able to finish all quests before they turn "grey"... you just level up too quickly.

Thank you, thank you! So, only quests green in color close to level range, check! About fast leveling, i honestly find it very hard to believe, since it is a slow and painful process, unlike previous titles...

It is a great game and all, but those features really put me off:
1) Enemies that you cannot kill (unless of a certain level) upon the moment of first encounter, unlike previous ttitles.
2) Merchants have limited gold.
3) Not all vendor types buy all things off from you.

It is a pain to have to "grind" your levels so much, in such an obligatory pace. And boy, xp sure is (still) slow in flow... And to travel here and there, loosing minutes of precious game time, just to sell something that you cannot do to nearby vendors...

there are many heavy XP boosts in the main storyline. i don't want to post spoilers... but in some you gain multiple levels ;)

you will reach the point where you get angry about not being able to complete quests while they still give XP... believe me.