Well, sure, players can do things to adjust to game specifics. But why a hell any developer would create a system the players need to do something about, or just to suck it up and go with the program?? In RL we sure know how to deal with sloppy performance in different areas to minimize our unhappiness but it is no reason to simply accept it. The same is with TW3. What was this great idea behind these multi-colored quests, which does not even adjust to story progression very well? Why not just have the set XP rewards? They can suggest a level when this quest should be attempted, but it is completely independent from XP.
But see, to me, this is the exact kind of thinking that over-inflates something that really isn't an issue. The quest being green/grey in your log simply indicates the level of the opponents you'll come across relative to your character's level. The content and story of the quest doesn't change at all. The color coding allows the player to know whether the opponents will be on par or if they'll take 1 less Igni or a couple fewer sword swings to strike down. Not having the color coding would mean the player would go into each quest not knowing how their current level relates to the level of the opponents for that quest or you would very annoyingly have to check "recommended levels" of each quest to obtain this same information you can get instantly from the color coding.
Furthermore, you have to consider that if each quest gave a set XP amount for completion then the end-game would be completely screwed up for so many people. You have to think of it from a marketing perspective. Many players just want to get through the main story and aren't interested in experiencing all the nifty sidequests and whatnot. This person may be a parent, or game reviewer, or some other individual who can't dedicate hours upon hours to a single game. So these players focus almost exclusively on main story quests. Now, take a player such as myself that refuses to leave an area until I've searched every nook and cranny and have done every single available quest. How the hell would you balance the final boss fight to accommodate both of our playstyles? Since I did 200-250 more quests than the casual player and each quest gives a set amount of XP, I would be umpteen million times a higher level than the casual. You can't make the last boss unbeatable for the casual player, so the boss can be challenging but must be simple enough for the casual to have a chance at beating him. Yet if that's the case, a player like me would one-hit-kill that same boss with my eyes closed. What a boring, anti-climactic, and overall crappy experience! The W3 XP system is actually a genius way of making sure that regardless your playstyle your end-game content will be fun and challenging. Both the causal player and myself will be roughly the same level when we enter that final boss fight and have roughly the same experience. I find this method to be far superior than a method that would make the final boss impossible to beat for some players and a complete joke of a challenge for others.