sorry forgot about that question.. the world today? environmental change, diffrent weapons, difrrent clothing and footwear (depending on the martial arts origins). and attackers using diffrent fighting styles/tactics.
most fights come about in a city environment, cities today are alot diffrent than cities 200 years ago.
over I don't think there is much diffrent in any one catagory but over all there are changes, perhaps not enough to support an argument that simply because an art is old it's not effectiveanymore, however probably enough to show that some arts have become less usefull due to environmental change.
I think the biggest change is the availability of statistical and scientific information.. 200 years ago you couldn't get a report saying the most common forms of fights and the most common tactics. Another big change would be fighting style. most fighters you run into arent' going to fight in an "easternstyle" rather they are going to have some kinda of boxing/wrestling type of combination with maybe some low soccer kicks in there.. for various reasons it's kinda un common to find a TKD master that wants to mug you

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social diffrences might be there also.. if someone wants to attack you with something like a knife, they are probably gona get realy close to you before letting you see it, and because of sociaty today if someone walks up to you with a smile on ther face you may not expect a fight so you may not back away.
Legal system is somewhat diffrent, as are security systems.. you can be caught on video beating someone up on the street than get thrown in jail.. You legaly aren't supposed to resond to a dangerious situation with any more than the force required to protect yourself/others, since this is subjective to a person/people that ween't preasant at the time of the fight there could be some serious reperations for say, breaking someone's arm that threw a punch at you.
Fighting could also be diffrent because of weapons, if you beat someone up there's nothing stoping them from going to get a gun than killing you.. just as an example (I don't know how common that kind of thing is) but there is always the danger that someone might draw a weapon like a knife in the course of a fist fight, and if you are in an environment that it's difficult to manuever in, like a bar for example than that could be realy bad (compared to on the street where you can always run away).
You are going to see a fundamental diffrence in any art where you don't wear everyday cloths to practicein.. for example when I go to my JKD class I wear swets, normaly I wear comewhat baggy pants but the swets offer much less resistance to movement than any other kind of clothing I would wear, so obviously in a fight outside of class I'm going to be slower due to the resistance of clothing.. this may not seem like much but it will deffenatly affect your ability to fight.. simularly if I were to practice without shoes on, my style of delivering kicks would change and if my opponant didn't have shoes on my awareness of some attacks would be lowered.
Over all I would say the changes that would alter fighting for self deffence the most are
1: your opponants fighting style.
2: your environment, how fights will start, how you can avoid them, legal reprocutions as well as weapons and weapons tactics.
3: the availability of scientific information, and the ability to analise many fighting styles/systems and compare them.
there is nothing that would specificly invalidate any particular fighting style, but anytime your environment changes your optimal fighting system/style will have to change.. I think today you have to do alot more with alot less.. somecases you have to be able to deffend against mulltipal attackers posably using weapons without the advantage of a weapon(dpending the circumstances) and you probably cant dedicate much time per-day to studying martial arts (relative to someone 200 years ago).