I've been to Japan a few times. I love the place, but it definitely is kind of weird. Here's a few tips and observations.
"Friendliness" - Depends on where you are. In most of Tokyo, it's like..."oh great another white guy" Totally blase. You will not talk to anyone. Just point at the pictures in the menu and grunt. People are nice, but it's out of tradition, not any particular affinity for you. However, if you get out of the city it's a different story. A friend and I were walking around Yokohama looking confused (We were--couldn't find Chinatown) and a very friendly man at a gas station came out and asked us if we were lost. He showed us where to go and everything. This will not happen in Tokyo. The friendliest people in Tokyo are the older people--they would talk to me etc. even if their English wasn't the best. You'll never have a decent conversation with anyone under 50 though.
Crime -- It seems to me, that street crime in Japan is at the very least totally invisible. I was amazed that I could walk for miles through Tokyo at 2 AM and not encounter any shady characters or a hint of a slummy area. It's truly bizarre. (And quite convenient given how expensive Taxis are) I don't consider pickpocketing a major crime--as far as violent crime and stuff like that goes...I don't think you have to worry.
As for the foreigners in Japan--even the African immigrant pimp dudes are cool. Since they speak perfect English, they were some of the only people in Japan I could have an actual conversation with. After declining their, uh, services, I often had some pretty good discussions with them about life in Japan. After spending a few weeks in Japan--you're desperate to have any kind of conversation with anyone since nobody speaks English...or if they do, they don't bother. So even talking to a pimp was cool.
Video games -- this is the important stuff! As others have stated, you *HAVE* to go to Akihabara. And not only that, you have to schedule 2-3 days there. Being a super nerd, I spend like 6-8 days just excavating the place every time I go. There's so much STUFF! There's also lots of weird porn and pseudo-porn that you can't help coming across when digging through Akihabara--definitely freaks out my western brain, but it's generally hilarious.
Osaka is a great spot too--Den Den Town is the Osaka version of Akihabara (which used to be home to a Neo Geo World...the sign is still there!). Osaka is a little more gritty than Tokyo (Tokyo is so ridiculously clean it's seriously unbelievable) but not any worse than your average American city. You can get better deals because it's not as well traveled as Akihabara. Plus it's a great excuse to get on the Shinkansen (bullet train) which is truly magnificent!
There are other game and toy shops all over the place--Shinjuku, Ikebukero has a nice Toys R Us, etc. etc. It's fun and easy to explore given the INCREDIBLE mass transit system.
Arcades are incredible. Very clean. Often organized by floor--so floor 1 is crane machines, floor 2 is all fighting games, floor 3 shoothers, etc. Very cool. Lots of retro stuff too.
Getting around - Getting around Tokyo is a snap. The further you get from the big cities though, the less English (or Romaji) you see. Get a Lonely Planet guide to Japan and Tokyo, and then use that to figure out which subway stops you need to go on. In most Tokyo subway terminals there's a sign with English versions of the Kanji for each stop. But the place where you buy your token is all in Kanji. So what I would do is memorize the first and last character of the stop I wanted ("Ok--the first character looks like a duck on fire, the last character looks like a christmas present under a table") and then look for those characters on the map where I buy my token. I used this for reading street signs too.
I know virtually no Japanese, but I can get around quite easily. At most restaurants the menus have pictures, or there's rubber food in a case in front of the place.
Cost - I don't find Tokyo or Japan as a whole that expensive. I stay at decent hotels, and eat pretty well, and don't spend a lot of money. The food is *FANTASTIC*...and the best part. NO TIPPING.
What I do end up blowing all my money on are games and junk like that. Which is fine by me.
The prices you see in Tokyo aren't THAT great--ebay has pretty much ruined bargain hunting worldwide. People know how much stuff is worth--with that said, in places like Akihabara you'll find stuff fairly cheap--cheaper than ebay at least. Unless it's super rare. Also, if you're buying new stuff make sure you scope out 5-6 stores before you buy. There is FIERCE price competition in Akihabara--if you buy what you're looking for at the first store you see it at, you'll be disappointed when you see it for %10-30 less somewhere else.