The ability to mark emails as read! - I like to take advantage of this feature with emails that take a while to load, for instance, some email advertisements that I sign up for. It's not something I can't live without, but it would be nice.
Push - I don't necessarily need it. I might not even use it, but I know that the majority of the iPhone's users would, which is why it needs to happen as soon as possible.
MMS - I know we may never get it, but I feel like the iPhone would be the first phone in which I would really take advantage of MMS. Besides, many of my friends don't have email on their phones... and if they do, they still send things via MMS. I'm in college, so MMS is pretty prevalent. I realize that many people WOULDN'T use MMS, so I'm not going to count on it. I'm fine without it, but I would like to be able to use it.
Copy/Paste - To be honest, I would very rarely use it, but it would come in handy when I got a phone number in a text and wanted to save it without calling it. Plus, without MMS, it would be nice to copy and paste those ridiculous codes. I understand that people in a business setting need this badly!
Deleting/Forwarding/Timestamp on Individual Texts - I hear a lot of people asking what we have to hide if we want to delete individual texts. I don't have anything bad to hide -- sometimes I just want to delete a certain text for whatever reason! I think I should be able to do it. (Like I said, I'm young, so my friends and I have most of our conversations over text. Sometimes there is stuff that is said that I would prefer wouldn't be in my inbox forever. And some of the drunk texts I receive... I definitely want to delete some of those...) As for forwarding, I hate those stupid SMS forwards and do not condone them, but I miss being able to forward a text to a friend to ask what they think. (Or forwarding those drunk texts I receive to a girlfriend so we can laugh about it!) As for timestamp, I just feel it could come in handy sometimes. I haven't gotten able to download 2.1 yet (my regular computer isn't working), but it seems like Apple has introduced repeating text notifications -- which was my only other complaint about the text messaging. Good job, Apple!
Video record - This will likely not ever happen, which is fine because I have never used it on my other phones. I have a digital camera, so I rarely even use the iPhone's still camera. However, because the iPhone is supposed to be an all-in-one device, I think a lot of people would really take advantage of this feature. Sure, the camera isn't wonderful, but it's a cell phone... and my cell phones with less impressive cameras have still offered video record. It would also be cool to be able to take videos with the iPhone and upload them to YouTube. That probably isn't even possible now, but I hope that is a possibility someday soon.
Bluetooth music transfer - My friends on Blackberrys use this all of the time. I get jealous.

I know we won't get this, though!
It would also be cool to have something like Blackberry messenger, but I won't hold my breath. I also doubt I would ever use this feature.
I realize that Apple probably wants to target business users, which is why I feel that most of these things won't happen. However, the iPhone is incredibly popular in my age group, and these are the things my age group use. I can live without them, but I would like to see iPhone have some of the little, simple features that my other phones have had.
To be honest, though, I'll be fine with my iPhone if 2.1 truly does fix all of the lags and the bugs. I'd much rather them fix the important stuff before adding new features.