Levels
Why?! WHY WHY WHY is it the only good way to show an experianced hero is levels! Shouldn't better armor/pets/weapons/** and higher skill points do that already! I need your opinion on this one, I see no reason to use levels, but if you can convince me!
Smart NPCs
How smart should NPCs(friendly) be? Smart enough to run when monsters are coming? does that defy the point? They shouldn't have a negative IQ like in every game ever!
(non friendly) All depends on what they are I guess, civilized creatures should not have a negative IQ euther, bears/etc can though.
Collecting recources
Honestly, would you spend time in a single player RPG mining ore and making armor? Maybe if you were a hardcore gamer, is this an important enough issue to require attention?
Climate
Which would you like to see more of in a world, grassy plains, forests, barren deserts, scorched earth, snowy mountaintops? There has to be some of each, but I assume players would rather see more nice looking scenes than scorched earth and dusty barren deserts, again I need your opinion!
Holidays
Should there be holidays in game? They celebrated Christ's birthday in medevil times, and they had the day of the dead, not sure about any more. I could also make up holidays to go along with the lore...
Quests/Events that only hapen on certain holidays? Items only available on such days?
Item based advancement has problems too. What if they don't find your Sword of Uber Hacking? What if they do? Will both of these characters has a fun time, one that's challenging but not overwhelming?
Smart NPCs tend to be a "nice if you can have it" feature. Assuming there is time/resources to implement, it makes a small selling point. But it also has to be tested thouroughly to make sure they don't do something really strange/game breaking.
Collecting resources is another take it or leave it feature. Most games implement it as a parallel system, where not doing it doesn't seriously harm the development of your character. So the player may get an item that is more powerful than what is available to them at the time, but there is usually something even better out there that doesn't require resource processing. (An exception to this tends to be endgame items, which are there mainly for people to collect after beating the game).
Climate really depends on your setting. If it's supposed to be a harsh world where life barely has a toehold, then verdant forests just don't work. In general, as long as the environment has interseting things to see, it doesn't really matter if it's a desert or a mountaintop.
Holidays should never be realtime except as an easter egg. And given how easily some people take offense at such things, it may be best to not use the same holidays we find here...in other words, a winter festival is fine, but it shouldn't be called Christmas. Most RPGs use holidays as an event for story purposes rather than any sense of atmosphere.