This clip indicates that it is just a wall. And admittedly, what you want to look at may not be right in front of you. But why would you ever just have a wall showing?
This part of First Contact always seemed jarring to me. I'm not going to provide a list of examples but, empirically, every time I've seen the viewscreen on the bridge of any Hero Ship in a Star Trek series, it's shown a generic starfield. To me this implies a default view of looking forwards, with the command "on-screen" suggesting a re-focusing, or re-orienting, towards a specific goal. This may involve panning and/or zooming.
I have no evidence for this. However I also cannot think of any other situation in which the viewscreen simply showed a blank wall, and I agree that this would be a somewhat ludicrous default view, if only psychologically. I mean, c'mon: being in the middle of deep space is bad enough, right?
Even more vaguely speaking, I can imagine a writer's meeting involving a discussion of that First Contact scene, with someone saying "c'mon, we have a movie budget now; let's show the screen activating!" then someone else wondering out loud how to visualise that. If you just transitioned from a starfield to a battle then the audience is going to wonder what the frak view it was previously; if you have the battle up already then "on-screen" doesn't make any sense. So I do see this line and the accompanying visual effect as, essentially, a pandering to movie-goers. And a somewhat flat one at that.
Also, consider that not just Riker and Picard but also Data and Hawk were already practically gazing at the viewscreen before it was activated, as if it were already showing something a little more interesting than a solid grey wall. Frankly even this is enough for me to write this entire incident off as nothing more than a production frak-up.
Based on watching a random selection of TNG episodes (Deja Q, A Matter of Perspective, Tin Man, Best of Both Worlds Pt I), the answer is that the viewscreen generally shows an un-zoomed view out of the main window, a flat starfield if they're sublight and a moving starfield if they're at warp.
I'm writing this because although there's an accepted answer, it's not entirely correct.
The default view is actually based on the type of ship we're talking about - the ship's class. In the case of TOS, TNG, and VOY, the default view is a starfield, representing the forward view from the ship. It's supposed to function as a window would (and for all extents and purposes, may actually BE a window, although this isn't proven in the TV series from what I remember.) This applies to the Enterprise C and D.
The Enterprise E is actually a special case. In the E's debut movie, First Contact, it was decided that instead of a screen - the window-type thing we're used to seeing - the main viewer would actually be a holographic projection. As such, the main viewer wasn't always in use and would actually show the bulkhead behind it. That's why, in the referenced clip, we see the bulkhead before the image materializes - it's because the main viewer is provided by the holographic projection.
In the later films they scrapped this idea, claiming that the bridge didn't feel the same without the giant viewscreen as it was a distinguishing feature. Thus, we never say the holo-viewscreen again (and in fact, in Nemesis I believe, the screen is actually shattered - showing that it is, indeed, a window.) Edit: I'm wrong on this one. Troi crashes the Enterprise (again) and the front half of the saucer is destroyed, along with the front wall of the bridge, so this doesn't prove that the main viewer is actually a window.
In the Abrams universe, the Enterprise main viewer actually is just a window; it's very thick glass, and from the bridge, we can actually see the saucer hull (which is different than all of the previous portrayals of the main viewer). Images are superimposed onto the glass.
So to answer the original question: "Why would you ever have a wall showing?" This was a deliberate decision for the Enterprise E in First Contact; it had a holoscreen that was deactivated when not used. No other ships had this feature, and the Enterprise regained its traditional viewscreen in later films.
Edit: There's a link to Memory Alpha (which is better than my limited memory) here, which explains the main viewer types better than I did. http://en.memory-alpha.wikia.com/wiki/Viewscreen