People have said some really good stuff in this thread!
Here's my own personal experiences/observations in handy "Top 10" format!
1. Prejudice. That's definitely the number one. Some people really don't think that just about everyone now treats women with respect. Wrong. Chauvinism is alive and well and the first time you experience it will more than likely not be the last time. And this covers a lot of topics and territory from really awful things like inappropriate sexual advances to simply assuming you don't know anything about a certain topic simply because you're female.
2. To me, it always seemed like there was some secret club going on with females. Like even if they didn't really like another female, she was still in that club and would be pulled into the fold in certain situations where the scales needed to tip in a female direction. I've observed this first hand in work place scenarios where I was the outcast - unless of course the females were "teaming up" against a male manager or something, then all of a sudden I was given in the "secret handshake" and told of the "plan". I'm sure something like this exists with men (see #5), but what I've experienced with males more is simply just more of a, we want to get away from the females for a time and just go do "guy stuff". It didn't seem so, well, I don't want this to sound quite this bad but sneaky? lol
3. Help. Some men seem to think women need help with everything. This is especially the case if you are an attractive female. I never had to personally deal with a lot of that.
4. Being referred to as, sweetheart, babe, honey, hun, etc. etc. by men you don't know. Yes, this happens. It used to really get to me if it ever happened. People that won't use your name (but use some "pet" name like this) who you AREN'T sleeping with are most often times jerks. There are plenty of more appropriate ways to refer to a women if you don't know her name ... miss and ma'am are two of them ... how about asking for a name in a polite way? That too would work. Also the use of terms like that always made me feel "lesser" ... not sure how really to explain that but it was a just a feeling like, I'm sure that guy wouldn't have come up to another guy and said, "hey honey, do you have time?"
5. The "boys club". You're not in it anymore. If you're friends with a bunch of guys and they're all going out to the bar and they don't invite you, it's because they don't want you to come. Probably because they're going to get drunk and crack dirty jokes. Sometimes, you just have to leave the boys club do it's thing and not take the non-involvement personally. I've known a lot of women who have. Especially if they're in a new relationship with someone. If they're not invited to go somewhere they start to get all emotional and think up all these crazy scenarios about why they weren't invited. I've actually been with a group of girls before when someone's boyfriend doesn't invite them somewhere and seen it go from, "omg he didn't invite me!" to "omg he's having sex with 3 hookers right now!" Amazing.
6. These all have kind of a negative spin so far huh? lol Honestly, most of the time people will treat you nicely. So be prepared to have the occasional door held open for you and other stuff like that.
7. Someone else already mentioned the long wait at a public restroom, but also be prepared for women to potentially talk to you in there or to overhear other women talking. I always thought that was the weirdest thing. Like you're in a public toilet, in a line, ready to piss your pants, and some women says something to you. Or you'll hear two women carrying on a conversation while they're sitting in the stalls. When I need to use a public restroom I just want to get in and out of there not go to a social club. lol
8. This kind of goes along with #2, but be prepared once other women accept you into their fold to have to talk "girl talk". We're talking that sort of stuff like talking about periods and other general TMI topics. I liken this to "guy talk", which is usually just tall tales and bragging about sexual prowess but can sometimes get to places like, "Dude, you ever had hemorrhoids? Man I got this crazy one right now ...." lmao. Same kinda thing, only expect it to be about "female issues" a lot of the time!
9. Workplace bias. I could easily lump this into #1 but I thought it deserved to be it's own point because I know so many women that deal with this. There is still a lot of preferential treatment going on in the workplace. Many times a man will advance up the corporate ladder while I women remains an "admin" (a catch all secretary). I have two friends in this situation now and they're kind of stuck because the job market is so bad right now. And no matter what people say, men in some professions are still making more money than women in the same professions. I've also know women in corporate environments who were very talented with a lot of great ideas but were continually snubbed at meetings. It's also very easy for employers to skirt around issues of bias by claiming that someone was "more qualified", "more experienced", etc. etc. So be aware of situations like that in the workplace and be vocal about pointing them out and about your skills and qualifications.
10. Go to a day spa. Seriously. Total experience in and of itself. lol