Understand what you mean about VBox and the extensions, but to be fair
it's not an Ubuntu issue as the same applies to VBox on any platform
and it's VBox who chose to do it that way. Used to be there was a
different set of guest additions per platform but they've all be
rolled into one common installer. Maybe future editions of VBox will
do away with this, only time will tell but it's VBox's call on that.

However, once you have the extensions installed then it does help with
the running - been a VBox user for some time now :-)

Accessing USB devices using VBox and WinXP is a two step process, plug
into the PC and Ubuntu will read it by default and to get it to read
in WinXP look at the bottom right of the screen and Right Click on the
USB icon then click on the USB device you want to use - this will make
it visible to WinXP and is effectively the 'plug in' step to it. When
finished, use "safely remove hardware" in WinXP as normal then Right
Click on the USB icon as above and click on the device. This is the
same as unplugging from WinXP and releases it back to Ubuntu. It may
sound complicated when written like this but it's actually very quick
and simple to do.

Accessing Ubuntu folders in WinXP is by using 'Shared Folders' in the
WinXP settings in VBox. Once a folder is shared that way you can then
use WinXP's 'Map Network Drive' to create a link to it and assign a
drive letter. The shared folder appears under 'Virtual Box Shared
Folders' when mapping. It's possible to share Ubuntu's 'Documents'
folder then change the path of 'My Documents' in WinXP to this to have
the two permanently linked - i.e. items put into 'Documents' in Ubuntu
appear in WinXP's 'My Documents' and vice-versa.

Not all users will end up running Virtual Box and WinXP, they'll just
use Ubuntu and in that respect it's as easy to use as Windows IMHO and
in some ways easier. Installing programs from CD is the WinXP way,
Ubuntu does not do this and isn't set up for it - installation is by
download and in this case a .deb file auto-runs the installer without
any need to consider permissions, and of course the Ubuntu Software
Centre makes the process even easier

PS - you have remembered to add your user to VBox Users ? That's
essential to get sharing and USB access working and again may well be
automated at some point. I found that I got an error saying that USB
devices cannot be accessed until this was done even after I'd done it
and USB was working fine, just tick the box to not warn of this again.