After the OS, OfficeRT and a bunch of apps, you will still have more that 20GB. As you say, you can always add an SD card and while you cannot add apps there, you can get music, movies and photos there.
The one specification where the Surface lags far behind the current industry-leading iPad is in screen resolution–the iPad features a “retina” display, but the Surface only has a 1366×768 resolution. However, according to the Surface engineers, it shouldn’t be a problem–higher resolution actually makes the human eye see less detail. I’m not convinced, but the engineers certainly are:
Screen resolution is one component of perceived detail. The true measure of resolvability of a screen called Modulation Transfer Function (MTF), not Pixels. MTF is a combination of both contrast and resolution. There are over a dozen subsystems that effect this MTF number.. Most folks just focus on one number out of dozens that effect perceived detail. Without good contrast resolution decreases. Check out contrast sensitivity of the human eye graph.. Basically, as resolution/DPI increases the eye has becomes less sensitive.
There were a good deal of unanswered questions. Notably, we still don’t know whether there will be a way to avoid “double-dipping” when paying for Office, and why the smart cover is more when bought separately than when bundled. Still, if you’re interested in Microsoft’s new tablet, there’s a wealth of chatter in the AMA.