There are a couple of things I can think of now and some have already been covered off on here
First off the Gmail Application itself.
I have my Gmail account pickup mail from a secondary Gmail account and also an ISP provided account. Now on the PC (Via the web portal as I don't bother with email clients on the PC) you can set up gmail to retrieve mail from other accounts but also send via those addresses also.
On the G1 you can't which is a minor shame, It would be cool if you could at least reply to an email sent to one of the other accounts from that address but you can't it defaults to your Gmail address. It's not a deal breaker for me and it doesn't cause me any issues as I have the G1 set up with my main email address anyway but it would be a nice to have for the future.
Google Reader.
The web portal works well on the phone but damn I wish they could build some kind of simple application for the handset itself. It just feels like there is something missing from the mobile web version and more functionality could be added I feel on a handset version.
Google Docs
Ok to be able to edit etc for the future would be good but my main little issue is that I can preview when I receive documents via email but can't download. Ok so you can't edit yet but I would have liked there to be a proper docs viewer on the handset that would also allow me to save the attachment and then at least upload to google docs (Which I use often) I except that I can do this when I get back to the PC but would be nice to have the functionality on the G1.
Picasa
Well I don't think I will be taking many photo's with the handset as it currently is as well lets be honest the camera isn't up to much but yeah why not picasa functionality would be good but there are applications in the store that can do this anyway so no biggie to be honest.
As I said none of these are deal breakers for me and it hasn't stopped me enjoying the handset or getting the most out of it, it's more musings and hopes for the future really.
Unless I have missed something though it would be nice if Google set up a wishlist site as Nokia did a couple of years ago for s60 where people could post up suggestions and comments as to what they would like to see, but I am sure they take very close notice of forums like this anyway so hopefully they are reading and taking little notes
