I'm the last person who will tout Isshinryu karate as a superior method of practical self-defense over others. As I have said here more than once, the style one learns is only as good as the quality of instruction. The following is a short list of some of the stock selling points of Isshinryu
Techniques adhere to natural body movements.
Vertical punches are favored over rotating or "corkscrew" punches as the former are considered to be stronger, faster and more injurious to the receiver.
Strikes are almost always delivered in a snapping (as opposed to a thrusting or pushing) fashion for the same reasons as above.
These are the major features and realize - especially if the reader is an Isshinryu person - this list is hardly exhaustive. You can find this info anywhere online or hanging on the wall of most Isshinryu dojo. These tenets have existed in Okinawan karate in various guises dating back to about the eighteenth century, but the founder, Tatsuo Shimabuku, considered these particular ones paramount to his system.
Here's another list of items that sets Isshinryu karate aside from some of the other styles of "traditional" karate. These things are rarely mentioned, let alone advertised like the ones found on the first list
There are a total of eight karate kata (pre-arranged forms) in the system. No empty-handed forms are taught after black belt, only weapons (kobudo), of which there are seven. This is a major departure from most karate styles that require students to learn literally dozens of forms, many of which are pinan (beginner) kata, none of which appear in Isshinryu.
Isshinryu does not endeavor to perfect the trainee's character or make her/him a good role model or citizen, per se. No lofty aims aside from prevailing in an unpleasant and possibly horrific altercation are sought.
Isshinryu techniques are not set in stone, but subject to innovation as needed. Shimabuku, taking a line from The Eight Precepts of The Fist ("Adapt to changing conditions"), sought to advance his art through effective modifications as the opportunities presented themselves.
I like the idea that there are only fifteen kata in Isshinryu. That's still an enormous catalogue of techniques. There's an old saying that it takes a lifetime to master a single kata. One style of Okinawan karate has over fifty forms in its kata canon. Somebody's wrong here, big time. At least Isshinryu found a middle ground.
None of this is an attempt to one-up another's karate style. Shimabuku would tell his students "All bottles are good; there is no 'best' bottle", a metaphor to express that there may be differences found in karate, but each serves their purpose in their own way.