I almost didn't say anything, because Simon's "don't ask us, ask your users" is great adivce. It's a question not asked often enough by builders. I do have one suggestion though that probably fits in just about any mapping application: ability to query or identify location.
For reasons I'm not clear on the universal question "where is that? {stabs finger at screen}" is often missing, from both web maps and desktop mapping applications. Reporting the mouse pointer or crosshair coordinates in the status bar or a text widget somewhere is insufficient. Usually the next step after getting an answer to wheres-dat is to do something with that information. So don't just report location, give it. If the user has to hover the mouse pointer and then copy numbers down, the tool has failed.
That's the bare bones functionality. Making it more useful might include things like multiple coordinate systems/formats - degrees minutes seconds (131w33'22" 61n24'12"; 131:33:22, 61:24:12), decimal degrees, UTM, ... or geocoded (4024-4rth Ave) or relational (500 meters northwest of bus stop #345) if they're in an urban area.
Snapping to the nearest object is another feature to consider.
Also be kind, remember the last X locations which have been queried. Perhaps look for relationships between them. And so on, I'm sure there are other useful features I'm not thinking of right now.