I agree it's best to have a loose itinerary.
ideally, i want to get through germany(berlin, frankfurt), italy (florence, venice, rome) for sure.. as far as france, i guess i want to get through (paris.. is it overrated?) and spain (not sure what city) and maybe UK (london)
Alright, if I was you and wanted to see all of this, this is what I'd do:
First night, Amsterdam.
day 2 - Train from Adam - (1 day) Berlin (2 nights)
day 5 - Train from Berlin to (1 day) Florence (2 nights), Venice (2) Rome (2)
day 10ish flight from Rome to Frankfurt (2 nights)
Train back to Amsterdam, spend whatever time you have left there.
You get a semi decent amount of time in all the places that are a must see for you, plus there's a little bit of flexibility and time devoted to travel.
I would eliminate Spain, because you simply won't have enough time, unless you trade it for Italy. Spain is absolutely amazing though, so when you get the chance, go!
Edited to add: As far as Paris goes. Some say it's overrated, some don't. It's a hot tourist spot, so pretty much whenever you go, it'll be packed. But, there is a lot to see. For me, it was just nice strolling along the river around all of the people. We spent a lot of time in the Cemetary and walked around a lot. The Eiffel Tower was cool. I do think it's slightly overrated - there are a lot of very cool places in Europe. But, I would like to take an extended weekend trip there with my husband, just for fun.