None really, you just need to watch that they don't get an ingrown claw.
You can google the term and pull up some interesting sites.
Some people thing it make for a more dextrous kitty, so the can open and get into stuff that other cats can't, like having thumbs.
I have five polydactyl cats, and the only problem any of them have ever had is the small claws that some of them have between their primary toes. Those claws can be tucked so far down between their toes that they can be extremely difficult to clip, and they can get really sharp. If I see one of my polys start to limp, I have to get those smaller claws clipped so that they don't stab the sides of their toes when they walk.
Polys are very good at grabbing things if they have double foot pads. They can also climb trees like nobody's business.
Here are pics of some of the poly feet in my household. Noddy (light grey tabby) is poly both front and rear. His female cousins, BooBoo (darker grey tabby) and Pretty (black longhair) both have thumbs in front and normal feet behind. They have a third sister and a mother with all normal feet. Sweet Sweet and Siliman (male orange tabbies - not related, as far as I know) might be poly both front and rear, but I'd have to check their rear feet to be sure (I can't remember at the moment). I find it interesting that each of my three poly boys have more toes than the two poly girls.
Not necessarily. Noddy is massive. At his heaviest (morbidly obese), he was 26.45 lbs. He now weighs in at a too-lean weight of just over 16 lbs. At the other end of the scale, BooBoo, Siliman, and Sweet Sweet are all within spitting distance of 10 lbs, while Pretty is about 11.4 lbs.