At their basic the two applications are different in philosophy and really do seem to have divergent target audiences. Google Earth is a vehicle to location based services (for instance route planning) in 3D, whose primary target is the "everyman". World Wind is a scientist and student, programmer toy (in the best sense of the word) whose plug-in architecture means in the long run it could have more features GE as it is easy to add plug-ins and customized to fit its user exactly. For example one plug-in enables GPS plotting and another adds measurement tools (more primitive than in Google Earth or TNTmap) another adds a compass rose and for the artistically inclined another adds shine to the oceans, moons to the sky, dynamically updated cloud overlays , and realistic skies from ground level etc. Plug-ins are probably the coolest elements of this GIZMO called World Wind and they shouldn't be under rated a comparison which doesn't consider them is not giving WW its due measure.
In comparison Google Earth comes out of the box (so to speak) very nicely equipped but some of its features are paying user only features, however most of the ones your grandfather and aunt will appreciate are free built in and have better useability quotients than their closest equivalents in WW and its plug ins. (Ammendment: I have an incredibly savvy aunt who would be far more interested in the science toy features of WW, she obtained a CS degree while in her early 70's way to go Aunt Margaret)
Google Earths interface is approachable and many parts are quite a bit more refined than World Winds. WW gives you the control about what you see rather than letting the GE corporation decide... though kml based overlays and the ease of their creation make this not entirely true. WW can be smoother in loading/rendering its layers because cacheing is effectively only limited to your hard drive space. GE has commercially purchased imagery so these are often but not always better than the publically available layers in WW. Better can be relative for instance sometimes the artist in me wants consistant color / contrast and other things the patchwork of satellite data of GE doesnt provide. The management of Blue Marble next generation data in World Wind is sweet as the ground cover reflects the same time last year.
Google Earths labels and vector data overlays are well beyond the offerings in WW and with KML and easily created user defined hot linkable icons it is possible to take Google Earth even farther ahead in this arena.
World Wind has the underpinings which will allow loading of some common GIS formats a rudimentary method (without interface) exists for loading a subset of Shape Files.
Google Earths KML allows you to easily publish overlays and provide web links to them... no extra installation steps for the person wanting to view it! and this is very significant.
Google Earth and World Wind both have limits in this arena... a simple one is neither of them have implemented a GetFeature Info, not even via a plug-in at least not yet. Which leaves a lot of WMS layers rather hard to decipher, although other limits may be more significant
World wind actually comes with extremely basic wms support.(basic in terms of doing getCapabilities ) that could be upgraded considerably with a plugin called WMSInterface although TNTmap can also save layers as standard Add-Ons (configuration files) for World Wind.
Note Google Earth doesnt have any form of GetCapabilities ability of its own unless you buy a rather hefty upgrade to it and I havent investigated that version.(Ammendment : Google Earth eventually enabled a rather basic limited WMS support) Something like TNTmap can be used to generate a kml launcher and a reflector like the tntmapproxy.cgi to make appropriate kml at any given view. TNTmap when serving kml is something like a generic wms reflector where as most kml reflectors you will see are highly specific.Google earths network link requests do not provide adequate information about the view angle area for kml reflectors to choose proper resolution and view areas particularly a problem at low altitude views looking off towards the horizon.
Google Earths comparitive lack of cacheing means that it is continuously updating which for dynamic data that is actually a desireable thing. but for a smooth experience it leaves much to be desired to compound this problem google earths loading graphic for network links is tacky and garish (I have been able to replace this locally with a larger png with thinner borders and I find it much nicer) in comparison to the subtle linear elements in World Wind. World Wind also errors gracefully something vital when you are using data from diverse sources of variable reliability.
Google Earth handles PNGs (both 24 bit 8 bit and with translucent colormaps) and Jpegs rather well (as well as tiffs and gifs), World Wind is deficient in regard to PNG transparency and treats all image transparency as on and off. Google Earths layer management system is rooted in kml and stores display parameters,including display order and translucency there-in something which World Wind really needs to make in its add-on/config files.
Google Earth is now cross platform, a Mac Version is now official, ws. This may change in the future but for now World Wind is mired in the PC only world of windows.(update:World Wind has produced a JAVA module which expands its impact into other platforms however as this is very much a subset of the capabilities of WW it is hard to acknoledge it as really being WW)
This provides good support for OGC WMS and overcomes many problems in World Wind, but as these problems apply to Add-On layers in general within World Wind as well as those saved from TNTmap so they are still worth mentioning as limitations of World Wind.
World Winds built in layer manager while it allows layers to be partially transparent it does not allow this state to be saved which is quite painful.
Further its render ordering is annoyingly alphabetical??? within a category and again this category is changeable... but not saveable.
Layer ordering, layer specific cache management and navigating to layer extents and others many of which at some level really should be implemented in World Winds core functionality instead of only for layers loaded via a specific plugin...
WMSInterface is indeed a great plugin for World Wind even so it falls short of what it could be (remember the lack of a get feature info support).
The WMSInterface plug-in in its current form lacks support for current WMS standards 1.3.0 but works ok with 1.1.1, It is picky about naming conventions (this is not in accordance with the protocol) and appears to have problems if a layer name contains a space.
The image type of the overlay is lost each time the layer is edited defaulting back to jpeg... a slight annoyance as the number of features it includes are quite nice
The WMSInterface is published as a binary dll, with a creative commons license (quite unlike many of the World Plugins which are even open source). For most folks this isnt a problem though with the target audience of WW, the scientists, programmers and students, nope "sorry guys you can't help fix it!!!" I think this misses out on some of the strengths of WW. Even so... Perhaps one of the strengths of the open source World Wind is that the writer of such plugins can choose not to be open source or even sell a custom version of WW itself with task specific plugins pre installed if they wish.