Most difficult traits to play with ...

[Alexandria_]

I'd probably go with these five:
Hot-Headed
Unlucky
Absent-Minded
Clumsy
Grumpy

[RubyLovesSims]

Clumsy isn't that hard to play with, though, is it?
As far as I can see, it makes very little difference to actual game play, so I always to choose Clumsy, just for the funny actions

[Naga]

How about Absent-Minded, Grumpy, Star-Quality (easier stardom, more easily publicly embarassed!), Coward and Can Salute?

[Asleep]

What's wrong with Can Salute?

[n8thegr8]

Shy almost always provides a negative moodlet.

[RubyLovesSims]

What's wrong with Can Salute?

It's just annoying, I suppose.

[Lilygirl]

I once had a Sim child that had the Can Salute hidden trait and it was great for making friends! Even without charisma skill she would greet a Sim and then salute them for instant friendship. It was awesome!

[RubyLovesSims]

Really? How cool!
How do you get it, again?

[n8thegr8]

I think you have to be the son/daughter of someone in the military career.

[Naga]

Ok, scratch Can Salute off. Absent-Minded, Coward, Grumpy, Star-Quality and what else?

[ratchie]

I think you have to be the son/daughter of someone in the military career.

In my dynasty Turquoise has the ability to salute she is not the daughter of someone in the military but her her spouse was in the military.

Rachel

[Ausette]

I'm pretty tolerant of most traits (with Absent-Minded as the exception), but some just strike me as a waste of space. Never Nude would be the obvious one here, and possibly Vehicle Enthusiast as well. In addition to those, most of the 'anti-' or '-phobe' traits, such as Hydrophobic, Technophobe, Can't Stand Art, Hot-headed, Grumpy and Dislikes Children provide negative moodlets and effects without corresponding positive ones. Don't get me wrong; I like the variety and I appreciate the challenge of these traits (except Absent-Minded, which is annoying), it's just that some are more beneficial and playable than others. Did I mention I don't like Absent-Minded?

[Naga]

Well you can't say a trait is useless, because they all depend on the player's style.

Did I mention I don't like Absent-Minded?

For someone who hates Absent-Minded, you sounded like you have it!

[MasterSiwel]

For me, It would be the trait "technophobe". It makes things that were easy very hard.

[Naga]

Technophobe is good if you want a yellow ghost. It's also good for smashing all your enemy's electronics.