An iphone serves one main purpose, a cellular telephone. All of its other features are add ons and are pretty much offered in an ipod touch.
Where as an Ipad is geared and designed for a multitude of uses, while managing to limit usability as much as possible.
The ipad could use some ports (usb,firewire,dvi) to name a few, apple could even offer external cd/dvd burner reader drives for it.
The ipad is basically an all in one device without the "all", but may hit great sales if its market is people who don't desire much for technical ability or customization, and where the preset iphone os type of environment streamlines apps they use regularly, and the portability is a huge factor as well.
It is a better looking tablet device than anything out there now, and will be probably for a while though, even without things I desire.