Straight back legs

What are the downsides of having a horse that is straight behind? How does this affect their movement and capabilities?

There is less spring and this horse would have a rougher trot and canter bc of this. A century ago, a horse with otherwise good conformation and straight back legs would have been trained to drive instead of ride.

Posty back legs make for a very bumpy, hard to sit trot.

Straight hind legs will make carrying capacity of the hind legs diminish significantly. The horse will have a hard time with collection, which means that jumping and dressage (beyond basic levels) will be pretty much a no-go.

However, if you're pleasure riding and just doing a bit of everything with no huge aspirations as a sport horse, straighter hocks do tend to remain sounder - so there's a benefit :)

Really? I had never heard that straight hocks tend to stay sounder. Zulu is pretty straight hocked.

I had always thought straight hocks, while staying relatively sound themselves, caused other problems, including spavins and arthritis in the gaskin.

That's what I understood, Chiilaa - happy to be proved wrong though!
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A horse straight in the hind leg (posty) can be a horse who has stifles that pop out.. and that can be a problem. Posty hind legs also can lead to stress on the hocks leading to bone spavin.

This is in addition to limitations on movement as stated above.

Maybe just a little

Yep, *generally* there are less ailments to the joint in a straight hocked horse, than one with significant bend in the hocks.