My advice is to wear clothes that feel make you feel comfortable. And by that I don't mean wearing sweatpants or a moo-moo. lol! I mean comfortable as in you feel like yourself in them, and they're something you would wear at home when going out to a casual dinner with friends.
Jeans are great. I prefer them for evenings though when it is cooler. Usually, we trekked back to our hotel for a rest after a long day of sightseeing, and I changed to jeans and a nicer flowy top for dinner.
Almost all the hotels I stayed in throughout Europe had laundry service or could direct you to a local place that could wash laundry for you. Some hotels are more affordable than others though on this. For me, it was totally worth the Euros to have the laundry clean and to wait for me while I spent the time enjoying the sights.
I'm not a huge fan of the "travel" pants sold at stores like Eddie Bauer or REI. They are "swishy" sounding when you walk, and clingy when there's any kind of humidity. I actually really enjoyed a pair of linen pants even though they tended to wrinkle sometimes. They were flowy, comfortable and looked a little less awkward on me than the cargo style "travel" pants.
I'm not a person who normally wears dresses or shorts, and the pairs of those I did bring with me ended up getting shipped home about half way through our one-month trip through Europe. So definitely only bring things you would normally wear!
Pack clothing layers that you can add or remove based on the temperature. (Example: a cami or short sleeve top, plus longer sleeve top like a chambray and a waterproof jacket for the top layer.)
A light pashmina, scarf or sarong is a great way to dress up any outfit or add an extra layer if you are cold.
Ultimately... pack lightweight clothes you enjoy wearing and that make you feel like "you". 2-3 pairs of pants, 3-4 shirts and the accessories to go with is plenty. Bring tennis shoes or at the very least, shoes you can walk 6 miles a day in! It is nice to have one pair of comfy and cute sandals to wear to dinner and let your feet have a break.
Remember: You can always buy some clothes in Europe if you find your wardrobe is lacking.
Merino wool is awesome for socks... I haven't found it a necessity for any other articles of clothing though.
Lastly, contrary to what you might read - absolutely no one in Europe cares what you're wearing. As long as your clothes are clean, fit you properly, and are suitably modest for your current location (i.e. churches) you will be just fine. You will be one of thousands of other travelers the locals see each day, and I'm sure they stopped paying attention to what their international visitors clothing long ago.
A smile and a friendly "hello" in the local language will get you more friends than any clothing ever could.