Slow on the uptake...

[Marsh]

�not the sharpest tool in the box��.one slice short of a sandwich�� 

What's your favourite phrase for this unfortunate condition?

[LionNo4]

The lift go,s to the top but the doors don,t open.

[Clare A]

If they'd one less brain cell they'd be a plant.

'Pond life'. (Best delivered in a low, sonorous tone, not least because the person is likely to hit you if they hear).

He/she's 'a bit behind the door' is a well-known northern England phrase - as in 'behind the door when the brains were given out'.

[Marsh]

That's great!   Thanks very much, folks.  Thirty-one so far.

[Xollob]

All part of life's rich tapestry.

[ursula62]

My dad used to say "He's an 0-2-0 tank locomotive".

Railway buffs might know that any loco with an 0-2-0 wheel combination would fall off the rails!

[Xollob]

He/she's from the shallow end of the gene pool.

[peanut]

a few fries short of a happy meal

[peanut]

or "hes as sharp as a bowling ball" or as useful as a solar powered flashlight or as useful as a rubber crutch.

[Xollob]

Found some lists here, here and here.
One of my favourites is "a few ears short of a bushel".

[Marsh]

Fantastic, Xollob!  I guess that just about wraps it up.  

Thanks very much, everybody.  It's been great fun reading your postings.

[Philtaz]

He/she has the intellectual depth of a car park puddle.

[secretspirit]

If he had a brain he's be dangerous!

[secretspirit]

Sorry HE'D be dangerous

[NinjaSleeping]

Not the sharpest tool in the shed