Some people like shortcuts, some people prefer using menus and the mouse. For the people that like using the shortcuts, Lightroom is full of it. The last I checked, the Adobe techie where at 300+ shortcuts. Every shortcut can be executed via the menus or some context menu. Here are the shortcuts that I use on regular basis, pretty much every time I use Lightroom.

Key Purpose
G Switch to the Library Grid view
E Switch to the Library Loupe view
D Switch to the Develop Module
R Switch to the Crop in the Develop Module
P In the Library view, flags the selected images as flagged (Pick)
X In the Library view, flags the selected images as Rejected
U In the Library view, remove any flag from the selected images
Tab Shows or hides the left and right panels
F5-F6 Shows or hides the left and right panels
F7-F8 Shows or hides the top and bottom panels/filmstrip
1-5 Assigns a star rating from 1 to 5 to the selected images
0 Removes any star rating to the selected images
6-9 Assigns a colors to the selected images
B Adds the selected images to the Quick Collection (the selected target collection)
Ctrl / All the available shortcuts for the current module (Windows)
Cmd / All the available shortcuts for the current module (Mac)