Hello :]
I have not had the opportunity to attend a Japanese anime convention, but from what I understand, Japanese and American conventions are quite different. Some of it may be a little bit of a culture shock.
From what I understand, Japanese cons are mostly about buying and selling artwork. You can do that at American cons as well, since we have both a Dealer's Hall and Artist Alley, the difference being that the former is for official merchandise, and the latter is for fan-created merchandise. (It's almost never doujinshi and only rarely original artwork and comics, at least at Fanime, and usually fanart prints, charms, buttons, and craft items.) American cons tend to be more about both events and the social side. Most American cons have panels, where either invited industry guests or regular attendees will get up on stage to to Q&A sessions, talk about their work, give informative lectures, or perform comedy shows. Most American cons also have video gaming (Fanime has a fairly extensive arcade as well as console gaming) and video rooms, where attendees can watch anime or Japanese live-action films, or take part in the AMV contest. Fanime also has 24-hour programming, with late-night events including karaoke, anime screenings, a dance, and hentai screenings. Western cons also have a cosplay competition (the Masquerade), where people perform skits on stage and can win awards for their costumes and performances, and have cosplay gatherings, which I'll talk about below. I'm not sure if Japanese cons have these things, but I'm listing a few things so that you know what's out there and can compare.
Buying your badge/admission to the con doesn't have a long line (anymore), but some events, such as the Masquerade and the concerts, as well as some popular panels, can have long lines to get in, so be sure to plan accordingly.
The biggest difference is probably the cosplay culture. From what I understand about Japanese cons, it's seen as rude to wear your costume on the way to or back from the convention, and the cons provide changing rooms for people to get dressed. American cons don't provide changing rooms, and most people get dressed in their hotel rooms or at home before coming to the convention. It isn't seen as rude to leave the convention center in costume (and you are free to leave and come back at will, since your admission is in the form of a badge that you wear all weekend), and the entire downtown area seems to be taken over by people in colorful costumes. (I would encourage leaving the convention center to get food from nearby shops and restaurants instead of buying it all at the convention.)
Taking photos is also a lot different. It is perfectly okay for people to ask for your photo anywhere, at any time. While it is seen as rude to stop in the middle of a crowded area for photos (and you should move to the side), there are no regulations about when or where photos can be taken. Some events don't allow for photography or certain types of photography (no flash photogrphy during the Masquerade, no photography during certain panels), and the Cosplayer Hangout room, which has areas to sit and a place to repair your costumes if they break, does not allow photography. Otherwise, people can and will ask for your photo anywhere in the halls, not just in designated photography areas. Some areas of the convention attract more photographers than others, such as the area in front of the convention center, where you may end up having a small mob of people around taking your photo, but nothing at Comiket levels. Off-site photoshoots are also common, which you can set up ahead of time with photographers, or sometimes a photographer will see you and pull you aside to a blank wall or an area a block or two away for some more photos. Also, just because someone is allowed to ask for your photo doesn't mean you have to accept. It's also generally considered rude to take photos without permission, but people do it anyway.
Cosplay gatherings are another event that a lot of people attend. These are set up ahead of time, and are where people cosplaying from a particular series or with a particular theme will all gather at a single location at a specific time to mingle and take photos. Usually these are set up where there is a list of characters that the organizers go through, and when they call a character (or pairing, group, etc.), everyone cosplaying that character goes to a designated spot, poses, and lets people take photos. The organizers will then count down, and once they reach zero, everyone relaxes, goes back to the crowd, and the next character or group is called. In between photos, people will chat with other fans that they have met. This is a good place to make American cosplay friends who have the same interests as you!
Besides cosplay gatherings, as already mentioned, people will have small gatherings that aren't tied to cosplay or aren't official to the convention. These are organized on these forums, on Facebook, Tumblr, and other social media, or are private to a group of friends. Sometimes these will have a theme (for example, fans of Utena all going to a bar together one night...and now I need to organize an Utena fan bar meetup), and sometimes they will not. Since this is a con that goes on for 24 hours from Friday until Monday, a lot of bar-hopping and room parties happen. This is actually increasingly becoming a party and hangout con, which is perfectly fine, but I don't recommend going to the con itself while drunk. Keep that in your rooms. (You are also not allowed to have alcohol at the con itself.)
I hope this helps answer some of your questions more in-depth! If you have any other questions or concerns, feel free to post here or ask me directly. :]