Biggest disappointment is the lack of an EVF without a doubt in my case. You are going to sacrifice some in IQ going down to the smaller sensor, but you'll be amazed at what the IQ on the Q series can do. The biggest NON-disappointment comes in sheer fun and also the weight advantage to carrying a Q7 and a couple of lenses. Way lighter and NO disappointment there
I have a Q7, Q, and the 01, 02, 03, 06 and 07 lenses, (and the K-Q adapter) as well as a K-50 with the twin lens WR kit and a couple of inexpensive extra lenses. I travel a lot for work, and like to have a camera with me. The K-50 while a GREAT camera, never makes those trips - it's just too much extra to haul. The Q7 goes everywhere in one of those little Optio bags. I find the fisheye 03 a disappointment (hard to get focus), and my 02 lens is ok but not great. The 01 Prime is absolutely STELLAR. The 07 is just a toy, fun for goofing, or it just stays on the Q body in case I want to take a super small cam with me. The 06 is also a terrific lens. I tried out an Olympus XZ2 on a recent trip too, and the menu system was harder than heck to get my head around compared to the Q and K-50.
It is said often that the best camera is the one you have with you. The Q/Q7 are easy to travel with, batteries are inexpensive, a couple of lenses - and you're good to go. It's hard not to do well with a Q7-01-06 kit. Again, the 01 prime on the Q7 is just amazing at F1.9, and the 06 stays at F2.8 for the full range. That gets you a lot of coverage in a small kit bag and limited weight - and it will easily fit in a jacket pocket or small pouch bag. Again, the 02 is ok, but I rarely use it because the 01 is so much clearer.
For around town, or nature shots, it is great fun to take the K-50 out. And the lack of a viewfinder on the Q7 IS a bummer (focus peaking is darn tough to get exactly right). For general purpose fun though - the Q7 is hard to beat. Also, don't forget the little knob on the front of the Q for changing to user set modes - that is a great thing to set up if you want different types of photos on the quick.
Just my $.02. Cheers!