Hmmm...Good question indeed.As a fan of anime i`d say "One Piece",but that`s not the first thing.
Katanas-Becouse i always wanted one (inbetween other reasons...)
Anime/One Piece-For obvious reasons already said (...and yes...hentai,too...thou i`m not that much fan of it)
Sushi-I never tried it
Guitar-Now,this demands some more explanation;Recently,i found out that I`m a proud owner of Suzuki E21T guitar,a first electric-guitar model that was made by that company in time period somwhere around 1963-1967,BUT I HAD THAT THING FOR ALMOST 10 YEARS,and i couldn`t find any info on its origins,or name,or even identity,for that matter.So you can`t imagine my happines when i finnaly found out what is it.Since then,one of my first associations on Jappan is that guitar.
Samurais/Ninjas-It speaks for itself
Weird things/Weird porn-Let`s face it,asians (NOT talking strictly about japanese)are a bit weird when it comes to some things,especially if we`re talking about the secound one mentioned here.Also to add,my intention when it comes to this part of post ISN`T insulting,but just calling things like they are.I like weirdness sometimes,but here and there, i`ve seen some pretty "WTF,man...just...WHYYY?" made/done by asians.
Culture/Discipline-Thing that you simply HAVE to admire to,when it comes to  japanese.Period.