Some highlights:
- Any panel can be positioned/sized however you like
- You can create custom named layouts for your own common workflows
- Fully extensible via Python plugins (create new UI panels, menu items, hotkeys, etc.)
- Anything you can do manually can be scripted with python
- By default, (although this has changed some) the more-frequently-used hotkeys are single-key. And of course it's all configurable and (if I remember correctly) they have presets for matching other apps like 3ds max / maya.
- searchable commands palette (press [space], start typing command name, press [enter] to execute command)
- on load / "new file": it loads [something like] "default.blend" instead of a hard-coded set of defaults, so that you can configure its definition of an empty file (which IIRC also includes the default layout configuration). To make it quick+easy to change, you can set with menu option [File]+[Save Startup File].
- Windows can be broken out and dragged to a separate monitor
- Lots of highly-specialized UI widgets. e.g. inline ranged slider that captures the mouse on click and adjusts the value as you move the cursor, has a "progress bar" to visualize the percentage, allows keyboard entry if you single-click instead of click+drag, has arrows on left and right that can be clicked to step forward/backward, has label inline with widget, and has description on mouseover
- Generalized interactive timeline for animation and sequencing
- Powerful node-based editors for shaders, textures, etc., with nodes for each stage of the pipeline and utility nodes to apply transformations (e.g. add a node for a source image, draw a line from the image's color output to the input of a "brightness" node, etc.)
And not only can you use it for the obvious 3D tasks, but it can be used as a non-linear video editor (including things like cross-fade of audio and/or video, video superimposition, textual overlaying, cropping, stretching, mirroring, color manipulation, option to use 3D scene output as video input, etc.), shadeless 2D raster/vector animator, 3D printing, and probably other things I don't know about.