well most of it is in the design, but then theres also strength of the frame and weight of the frame.
firstly you need to have a square crown, which is suitable for resting your feet on as this is the basis for a large amount of riding skills. You should also be able to stand up on the forks comfortably in a stable position.
secondly you want the frame to have minimal clearance around the tyre, both over the top and down the sides. Not only does this reduce the chance of you knocking yourself on the frame when you stack it, more importantly it provides a lower point of leverage, which gives you better control. If there is too much clearance it makes tricks such as coasting much harder to balance. Also it makes it near impossible to glide with your gliding foot resting on the fork, making stand up gliding out of the question.
Also you want the frame to have a long neck, covering most of the seatpost. This makes it almost impossible to bend the seat post, as only a few inches of it are protruding (depending on whether you change the seat height). Also this positions the seat post clamp (which is preferably the type that needs an alan key) under the seat where it can’t catch your leg.
on unicycle.com its very hard to find a frame that has all of these features. There are some that have nice crown designs, but the frame cuts off about 15cm above the crown. Also most of the frames (such as the Yuni) have about 5cm clearance over the top of the tyre.
if you look around there, they really dont have any really good quality frames, compared to the large amount of trials frames they have on the market.
Out of the ones they have, this one is probably my favourite, although its hard to tell how much tyre clearance it has.
there are some very very nice frames on the miyata japan retailer, that ive been drooling over tonight. The large range of freestyle frames is obviously due to the huge base that the sport has in japan, which isnt so much the case in US and UK (and australia for that matter)