“I have no experience with forum RPGs (just the pen & paper, and the video game kinds)”

Think of it as of co-writing something. Complex rules and dice rolls do not work well on a forum where GM availability is low, in my experience, so it's more like each player alternatively describes the actions of its character, controlling some PNJs to add some depth. Rules are only here to ensure that players do not do extremely frustrating things based on the atomicity of posts or the fact that other players are not there (like, you know, killing or mutilating players without their implicit or explicit permission, slicing planets, etc...)

“one method I've thought of for simplifying the character creation process is to choose your character's role, rather than its stats.”

Well, this is how it works on any tabletop and forum RPG I've played so far. Even in D&D, we choose the role first, then roll 6 stats and distribute them around STR, DEX, etc... how we want. On "class-free" tabletop rules like Shadowrun or Humanydyne, players even just choose a set of things which their character is good or bad at, based on a fixed pool of character creation points. This eliminates the issue of unfair dice rolls that can impair a whole character's life : you get what you've chosen.

“Alternative method: Use descriptors in lieu of bonus scores. +0 is "average", -1 is "slightly weak", +1 is "above average", +4 is "superhuman", and so on. ”

Many RPG books try to put descriptors and examples behind numeric bonuses and maluses in order to make them more easy to visualize for players. In Shadowrun, as an example, if I remember correctly, you master a skill from +0 to +5, where +0 is the basic human in the crowd (e.g. You know how to drive, not how to do F1 racing), and +5 is full mastery, the highest level of control on something which someone can aspire at getting through training alone (e.g. When you drive, the car feels like part of your body, you feel every vibration of the mechanic and can tell exactly where each noise comes from). All levels inbetween are pretty well described.

“By the way, have you looked into the FUDGE role-playing system? ”

Nope. When the rules I know of do not to fit, I tend to use my own (simplistic) homemade rules instead. But I've seen games which used a similar +/0/- dice roll system, just keeping compatible with "normal" dices. As an example, Vampire says that 1 is a failure ("-"), 2-4 is neutral ("0"), 5 is a success ("+"), and 6 is a success that you can reroll for extra awesomeness ("++" ?)

“OK, I think I get the concept. I've seen some forums where they have built-in random number generators for different purposes, but I didn't connect the dots to forum RPGs. As long as a GM is available enough to set difficulty levels for certain tasks, then that's one way to somewhat automate things. Of course, common tasks could have predetermined difficulty classes.”

Yup, I've seen this feature in phpBB3 too. However, it has some polish issues : the functionality is a bit hidden in the interface, and I have seen claims that the random generator is far from being perfect (as in 999 times one side and 1 time the other side for a 2-sided "coin" throw, which suggests some serious rounding issues). Nothing prevents people from deleting a post and re-posting it in order to re-roll the dice. And finally, I have some issues on a conceptual level : too much dice rolls tends to encourage one-liner roleplaying (especially in fights) and reduce the player's creativity, so I prefer to keep with strict storytelling for the most part, and it one day there's a need for stronger rules invent them at this point...

“You could also do away with DCs altogether using a system similar to FUDGE or the Vampire system you described.”

Actually, I think that both FUDGE and Vampire have something like difficulty classes. There's a score which you have to reach in order to success when doing something, which in FUDGE is reached through skill level + dice modifier and in Vampire or Shadowrun is reached by throwing as many dices as you have skill points and counting total dice score.

“One idea to implement this would be to have a "Roll Dice" tag, which when the post is submitted, would generate random number(s) for each tag placed in the post to determine success or failure, along with some special markup or some other mechanism to discourage shenanigans. ”

phpBB3 has this, but it's badly broken. Editing a post re-rolls the dices...

“Why not use some part of the unique ID of the post, like with NNF. I won't know what this post's ID will be until I've posted it. The previous post by theraje has an ID of #12vskt7akvvk, since this is base-36 (0-9 / A-Z), one could use some simple pattern to decide what the roll was, like "the first number in the ID", or the first letter A-T (20 letters). ”

Because I'm not a web developer, and rely on the work of lots of talented people who deal for me with browser inconsistencies, SQL injections, hosting hell, etc... :) The phpBB forum which I use does not expose randomly generated unique IDs publicly, though it's well possible that it uses some form of them internally. For more courageous people, that could be an idea though...