Hope you don't mind if I clarify.
The Surface is ironically hamstrung by its own high resolution screen. Because it has a 1920x1080 panel, if the Desktop is rendered at that resolution, all UI elements are TINY. (Remember, the iPad has a 2048x1536 resolution, but only 1024x768 points of content displayed).
In order to make things visible and touch capable, you must scale them up - the default is by 150%. The problem when you attach an external monitor is that the monitor then ALSO scales everything up by 150% which you really don't want because it makes everything huge. There is currently no way to define separate scaling on a per-monitor basis, and if you connect to a monitor regularly, you must delve into the Windows settings to change this every time you connect or disconnect.
Tablets that have lower resolutions, ironically don't have this issue because no scaling is necessary.
The external monitor issue is one thing, something Microsoft is actually on record saying they are aggressively pursuing a solution. But the bigger problem IMO is the scaling on the surface screen itself. This is the BIG issue consumers cry about when they say the desktop is not suitable for a tablet. It is confusing, because what is iOS but a desktop? But Apple have scaled their UI elements correctly, allowing for finger use and for use at arms length.
This is where we start to get into things I think MS screwed up on. Firstly they should have designed their UI to scale up correctly, things like hitting close, minimize, resizing windows, of course font sizes, buttons, etc etc should all have been reworked YEARS ago. I'm coming from a background of Windows Mobile and Windows CE, they did this crap 10+ freakin years ago, we'd have a pocket PC with these tiny little UI elements, MS just didn't get it back then. Back then it was excusable, even though Palm did get the UI right, still we cut MS a break, but today with iOS and android it is just inexcusable to have the UI scaled the way it is on the desktop. So what did MS do? The exact opposite, they virtually completely ignored the desktop and created a new "mobile" tablet OS, Metro, and in doing so confused and pissed off a lot of consumer. All MS needed to do was adjust their desktop and Metro would not have even needed to exist. Granted legacy programs would have had an issue, but the choice now is to write a totally new Metro program, where in my scenario a developer would only have to rewrite with the new UI interfaces. I'm sure this could be made easy for the developer, just as Apple makes it a no brainer for their developers.
The UI is very usable on the Atom processors with the lower resolution, and looks quite good, arguably just as good as the higher resolution on the surface Pro, but much more usable. This is something either MS will get with time, or they won't and they will phase the desktop out in favor of Metro, which is an abomination IMO.