I dont think quests can be screwed till affect hardly on your levels. Basically if you miss some quests in the early days, it wont actually affect much. Majority will finish laiken battle at around level 18 to 20. As for quest after battle tower, I dont see any ways for you to screw it. If you miss it, just do it, since you can pretty much access any where already. Just make a save before entering hall of echoes and all will be fine.
As for differences between easy and hard difficulties, I can give you example in terms of damage, hp and damage receive, as I just did a testing since I trying to do some reloading on malachite gems. Below are testing case which I done on lvl20, 21 imps in depleted ore mines with my lvl22 ranger, with the same equipments. The test is done with latest patched 1.03 installed.
Easy ModeNormal bow damage on imps: 140+ onwards till around 170+
Rank 5 explosive arrow damage on imp warrior: 600+ and instant death
Imp mage magic missile damage on ranger: 5 damage x 2 hit
Projectiles homing: Almost close to non, can side step to evade any imp range and mage range damage
Hard ModeNormal bow damage on imps: 70+ onwards till around 90+
Rank 5 explosive arrow damage on imp warrior: 350+, need 5 casts of explosive arrow to confirm death. 4 casts close to near death, and can be kill with 1 to 3 shots of normal arrow
Imp mage magic missile damage on ranger: 15+ damage x 2 hit
Projectiles homing: High homing ability. Can even hit me with side rolling. Dont even think of jumping up to evade, as 90% will get hit
Beside that, I notice in easy mode, mobs tend to be more generous, they drop loot very often.
Enemies cautiousness is also very high in high difficulty mode. I had ranger stealth rank 5, and sometimes range type mobs can attack me before even their name being display with my cross hair pointing at them. And they tend to spam skills very often.
Edit: Some updates. Level 20 bandit archers tends to spam explosive arrows on their 1st hit and tend to spam it wherever they can in high difficulty. It deals bout 90 damage on my ranger with 40%+ on range resistance. If unluncky, they can easily kill me with some critical explosive arrow
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. I'm all for not holding the player's hand but this is rediculous. Just because I chose to do things differently than designer planned or missed a few side quests, I got screwed on much needed exp. On easy this shouldn't matter as much, I shouldn't have to scour every inch of the map just to survive the next area. Yes on hard I can see this, I guess now that I know how to cheat the system I'll be much higher level next time round but still...