Honestly, to me, manual zoom or lever zoom, both are the same to me, cos I'm used to my equipments.
As for videos... I don;t take videos...
And regarding Dslr? I didn't compare FZs to Dslr in the 1st place. cos comparing them is like... Pretty obvious which one would give better image quality. BUT, when I had fz20 in its prime days, i feel that it is much better than fz30 to 50, why? As you said its the megapixel increase. I feel that panasonic is so wrong to increase the pixels while retaining the same sensor, cos result = noiser image and lost of details. So thus I understood this and NEVER bought another FZ - I just love my fz20 so much and feel that 5MP is enough.
If you want to talk about Dslr, well, it a different business altogether. I use my 5D now mainly for my job, F31fd for party/candid, and fz20 when I go travel.
FZ50 is a great camera, no doubt, but FZ20, which has 5MP, is just the right balance for me. For me, image quality > other attributes, as long as I understand the functions, buttons, zoom (got manual or no manual) of my fz20.
To each his own.