My own list of not-do-to's:
1-Don't make it a shooter. You can make the action look kind of like a shooter, but please leave target selection in and just let character skill levels and build-related RPG type stuff determine accuracy. I like shooters and I like RPG's, but that doesn't mean they'll form a digital Reese's cup when combined. I like coffee and beer but you don't see me putting them both in a blender and pushing "innovate" do ya?
2-Don't go batsh** with the graphics. A Fallout game's appeal comes from the gameplay, not the eye candy. The screenshots so far could stand a little doctoring up here and there, but for the most part it looks great. No need to go overboard with next-gen eye-popping framerate-gouging polygon orgies.
3-Don't bow down at the altar of high level caps. It's a myth that an RPG can't be good without a high level cap. All high level caps do is give a bonor to mathletes. A midrange level cap, like between 20 and 50, will work just fine, especially if you include a decent scaling system for critters and loot. Level grind is still grind, even if the schizophrenic gamers out there won't admit it.
4-Don't drown the game in self-serving distraction features. D&D games and player created content are horrible about this. They put in convoluded crafting systems, customizable player housing, over-developed forges, and all kinds of other extraneous crap in an adventure, and it pulls players away from both each other and the actual gameplay. Instead of teaming up with buddies and adventuring, Bolt Van Der Huge is adding upgrade after upgrade to his sword, farming 50 splinters to make one arrow, and arranging furniture in his treehouse for three hours. Much more fun than actually playing the game, huh?
5-Don't let players steal items from each other. Bottle caps and currency, maybe. But if I spend an hour battling a radiated manbearpig to get down to a room with a rocket launcher in it, I don't want some flaky douchebag waiting outside the door to yoink half my inventory. Player to player thefts should be either non-existent or extremely limitted. Also, vehicles and their trunk contents should be immune to theft. I don't want some prick hotwiring my highwayman while I'm in the infirmiry trying to sweet talk a ghoul nurse.
6-Don't neuter the super sledge. Bethesda dropped the ball with the Super Sledge in Fallout 3. They weighed too much, had no kinetic knockback/knockdown, and there were so few of them that repairing one to max condition was a pain in the ass. If I whack you in the head with a super sledge and a strength of 10, scoring a snazzy critical hit, I expect you to either fall down or fly away. I'll accept nothing less.
7-Don't neglect the Science and Repair skills. This is something where I think Fallout 3 was actually better than the first two (blasphemy!). Using the science skill frequently to gain access to security/lockdown terminals and the repair skill to maintain gear was very satisfying and made the skills very active and useful. There wasn't a lot you could do with science and repair in Fallout 1 and 2, and whenever I made a techie character they felt even less effective than speech characters.
8-Don't leave charisma so weak. In all 4 games, charisma is simply too weak. Virtually every other stat and/or their related skills have some kind of combat function, either through perks or whatever. But charisma is ONLY used outside of combat. Don't overdo it, but please add some minor tweak that will have charisma gain a combat role. For instance, enemies have a 3% higher chance to cower in fear or flee from combat for every point of charisma over 5. You could also add a handful of perks such as "War Face: whenever an enemy first detects you, they have a 50% chance of being stunned for 3 seconds (or 1 turn or whatever)". Anyway it doesn't have to be much, but a couple of charisma based combat skills like taunting, intimidating, etc would go a long way in evening it out.
9-Don't turn Tesla Armor into power armor. This is another thing that was pointless and goofy that Bethesda did with FO3. Tesla armor looked and played so much nicer when it was just a high-tech metal armor suit with awesome energy resistances.
10-Don't have books permanently raise skills. Reading Guns and Bullets should not permanently raise my small guns skill. A system like this in an MMO would make it way too easy for people to just take tag skills that can't be raised by books, and then farm like hell to buy books to raise their other skill levels. It would also astronomically raise the price and rarity of said books. Instead, convert skill books to magazine issues, and whenever a character reads one, it acts like a new issue, and for the next 24 hours or an amount of time based on their intelligence (short term memory), they get a TEMPORARY boost to the associated skill. You won't really need to change the names of any of the books except for Big Book of Science, which you could just rename to Atomic Physics Weekly or something.
Well that's it for now but I'm sure I'll come up with more unreasonable orders to bark at you.