(Yes...I could have written this in a single blog..but I have a short attention span and only 330ml bottles of beer)
In no particular order..here is some points from the Mendel90 build.
Spend the $$ and get the frame templates printed on a single sheet of paper! It took an age to get 4 sheets lined up square, not stretched, and taped together enough to survive the hole drilling exercise (..or the whole drilling exercise...either one fits)
Careful when attaching heat sinks to the stepper driver chips. The tops of the pins are pretty close to the edges of the aluminuim heatsink block..and they don't really like being powered up with a whole side all shorted to each other. Lucky I bought a batch of 10 of them...I have 3 left (you only need 4 for the RAMPS board)
When wiring up the Z motors, check out how to wire them in series, rather than parallel. The RAMPS board has header pins for parallel, but it works so much better in series keeping the Z axis steppers in sync.
Get your end stops sorted!..(seems a common theme from the CNC build)..especially those Z axis stops. Seeing your extruder try and plow it's way through the top of the print bed is not a happy thing. Be ready to hit that reset button!
Be prepared to chew through a lot of filament fine tuning the Z height. I can't stress how important it is to get the bed level, and the zero height set properly to ensure that first print layer goes down properly, without zits (lumps that will make the steppers skip when they get hit by the nozzle), and with enough coverage to make the print stick to the print surface enough to hold on for the rest of the layers.
Search, search, and more search...before you ask. It's a big community out there, and they have been doing this a while. Chances are that a noob question has been asked before. Go look for it, and you will find a wealth of additional information on the way.
That is all...for now.