Neutral traits

[Limejello]

Can anyone think of traits that don't make a character more or less powerful

for example fat makes him uncarismatic and slow but stronger and with more hit points.

[dindenver]

LJ,
  I think this is an idea worth exploring:
Every Trait should be like this. Like Alert Should give you a bonus to spot ambush, but make it makes your character mistrusted because they are always looking in different areas and avoiding eye contact (indirectly by looking all around). So that every Trait is neutral!
  Just an idea, good luck man!

[JoyWriter]

edgy - like above
focused - get ability to do things without distraction at the cost of narrative control/creativity
scatterbrained - the opposite, with "fools insight" but inability to keep on one thing for long
(dis)honest
intimidating
unobtrusive

There is also the big 5 personality factors:
Agreeableness - You think about others but find it hard to disagree
Extraversion - Energy but inability to sit still by yourself
Conscientiousness - Ability to stick to plans, but also desire to stick to plans regardless
Openness to experience - Mental flexibility, but can be related to schizophrenia, think hippy/beautiful mind!
Emotional Stability - Ok I'm not sure what the bad side of this is, overconfidence perhaps?

There five are supposedly statistically verified measures of human personality, so might be useful.

Spirit of the Century encourages developing those kinds of things, but in a more dramatic way so you might want to look at what people playing that have come up with.

I also considered doing armour that way, by adding "metal 5" to a character, which is obviously not good against lightning or when swimming!

Bear in mind though that as with any specialisation, a weakness is not a weakness if it never comes into play, so someone who slinks into the background and cannot get peoples attention looses all disadvantage if he can get someone else to do it for him. This is one reason the SotC system is very good, because disadvantages fuel the advantages directly, so people want to insure they come into play. It also runs a form of bidding economy to insure that big disadvantages fuel big (or frequent) advantages. Without such localised price setting systems you may find it difficult to balance traits for all games.

[TempvsMortis]

Just do what most people do when they want to do that: use a balance scale. Have an even number of traits, where one is diametrically opposed to one other. To increase one would decrease the other, and visa versa. That will end up happening anyway with neutral traits, because to neutralize them you must be penalized as well as rewarded for gaining in them.

[Marshall Burns]

Wuthering Heights Roleplay by Phillippe Tromeur; check it out.  (There's a good review of it in the Reviews section here too)

Sometimes you lose a conflict because you have too much X, sometimes you lose because you don't have enough X, and a high value of X is just as probable to help you as a low value of X.

-Marshall

[Mickey]

There is also the big 5 personality factors:
Agreeableness - You think about others but find it hard to disagree
Extraversion - Energy but inability to sit still by yourself
Conscientiousness - Ability to stick to plans, but also desire to stick to plans regardless
Openness to experience - Mental flexibility, but can be related to schizophrenia, think hippy/beautiful mind!
Emotional Stability - Ok I'm not sure what the bad side of this is, overconfidence perhaps?

There five are supposedly statistically verified measures of human personality, so might be useful.

Just adding to this for the purpose of informing the OP, the last factor "Emotional Stability" is also called Neuroticism, which for the purposes of RPG design (creating a "bad side") is more evocative I think.
For example, a neurotic character may be more susceptible to fear and other negative emotions (sadness), but would be better able to detect negative emotions in others
To use a real life example, it is well known that most comedians are highly neurotic, and that this susceptibility to negative emotions can actually inform their craft, since humour is often based on making fun of others' (or one's own) misfortune.