I just wrote a really long post that wasn't posted because I'm new and I linked to Metal Kid's IV calc, so I'll just summarise:
Based on the assumptions that the breeding mechanics remain unchanged from BW2, you can transfer over a good a father, you use power items and everstone effectively, your chances of breeding: (all "around" as it's dependent on whether you're going for hidden ability, normal, or whether it's irrelevant. The figures quoted are if you're going for a normal ability out of a choice of 2, using the fact the female has an 80% of passing her ability down)
A pokemon with 2 desirable IVs, good nature and ability are around 1/25
A pokemon with 3 desirable IVs, good nature and ability are around 1/100
A pokemon with a desirable hidden power type and power, good nature and ability - around 1/1600 - based on getting 70 power, desirable type, good nature and ability, also assumes you chose the parents well. people will have to compromise on this I think, settling for a power of 60+ will bring the chances down to less than 1/160 for example (cba with detailed probability calculations here)
Legendary pokes - lol, forget about shinies with optimal IVs for a certain hidden power type. Be prepared to compromise a lot on IVs and (if you need it) hidden power power.
The only people it will really effect are VGC players tbh. For casual playthroughs it doesn't matter, and competitive people outside of VGC can either hack or use simulators. Shiny hunters be prepared to SR.