First of all, I feel one of the most basic requirements of playing in a PnP group is trying to build a character that fits in with the rest. Anything that clashes with other people's characters and lessens their enjoyment of the game for whatever reason (too strong, too weak, clash of outlooks etc.) is a **** move in my book.
No, Dave really needs to be a fighter or wizard. Yes, clerics might be able to pull it off, but a real Dave the Dragonslayer either needs flashy and destructive beams of death coming out of his eyeballs, or he needs to be encased in several millimeters of Dwarven steel and wielding an arcane blade of wing sundering. Given that Dave is a wizard or a fighter, then any build (I'm assuming no D&D 3.X here, since that's practically a classless system, with all the options of skills and feats, and I know nothing of D&D4 or Next or...or...) will eventually be able to slay dragons. The only thing optimization will give Dave is simply being able to slay the dragon at an earlier time in the game, hence my statement that such a player is only attempting to be more powerful for their level than that they would otherwise be; that they are in fact trying to monopolize the game's dramatic moments, steal all the glory, etc.
I'll talk about 3.5 since it's the D&D edition I'm most familiar with: if a fighter (or paladin) Dave (which I agree is probably the closest to the typical dragoslayer image) wants to be a dragonslayer (which for me means 'has slain dragon(s) as part of his background and is capable of slaying dragons at the game table') he will have to optimize quite a fair bit, because the average fighter/paladin has next to no chance of dealing with even the most basic defense of a dragon (Flight). Much like building a simple 'master of stealth' in D&D 3.5 required a ton of optimization because the plain Rogue lacked even the most basic stuff (Hide in Plain Sight, ways to bypass Blidnsight or similar, ways around even the simplest magical wards like Alarm etc.)
Even so, regardless of edition (not sure about the very old ones though), D&D has worked with CRs. Not optimizing might not mean 'can slay dragons later', but rather 'will never be able to slay an appropriate CR dragon'.
It's a very fine line, that line between character optimization and intelligent character development, but I can usually spot it during character generation. In our last campaign someone showed up with a Missionary *** Tainted > Mutant > spend 200XP for mutation of choice > Wyrdling > choice of techniques = Compel and Mind Probe. It's all perfectly legal (the loophole not being closed by the Errata, though Into the Storm did close the loophole on the Witchborn) since the rules don't state that the character needs the prerequisites for those psychic techniques. See any problem here? The person playing the Astropath was immediately made obsolete and wouldn't be the party's premier psychic until the end-game, plus the Missionary is arguably the best anti-psychic in the game, or at least the most psy-resistant. That means the Missionary, who can usually challenge the RT in social situations, now outshining them (with the ability to Compel), outshine the Astropath in telepathy related matters, and outshine the Navigator in their ability to stand down hostile psykers. And that's not intelligent character design. That's pure unadulterated power-gaming. It's selfish game-play. It's not allowing other players to shine in their chosen career because you've outdone all their abilities in a single career. Of course, the player had a long and detailed rationalization explaining how his psyker became a Missionary, and it was all quite believable, and there's no way in hell I'd ever allow such a monster into one of my games and expect anyone to ever again take me for a serious GM.
And now I've probably offended someone who drew up exactly the same character, thinking their special Mary Lou was their own unique brainchild that nobody else had ever thought of before, which takes us right back up to the top of the page, where I was telling Traejun that sometimes you just have to tell it the way it is. In my defense, however, I'll point out that this is all detailed in my House Rules, which some of you might have read in my thread there.
Believe it or not, there are games where that kind of character will be the norm, and the guy showing up with a plain, can just do the basic stuff, Rogue Trader or Astropath is the selfish one because he's expecting the other 3 guys playing optimized characters to carry him through challenges.
Apart from stuff that simply breaks the game (as in makes it non-functional, D&D communities mostly called it Theoretical Optimization as it wasn't intend for actual play), most characters should only be judged 'too strong' or 'too weak' in comparison with the rest of the player characters. If everyone else is roughly at the same power level (whatever that power level might be) and the DM doesn't feel hard pressed to challenge them, where's the problem ?
Edited by LordBlades