DZH has been on a Thomson cruise (but he's up in the bath at the moment). From what he's said, it's a lot less formal than some other cruise lines. But, isn't it NCL who offer "Freestyle" cruising (which I gather is meant to be less formal anyway)?
Thomson use their own ships. They are much smaller than NCL ships will be. They may not cater as well for children (if that's a consideration for you).
I am old.
I loathe the large ships - and, yes, I have tried a few. For about 4 years I went exclusively on the QE2 until bastard parent company Carnival decided to flog her off to rot in Dubai. I then tried the QM2. Once. Absolutely soul-less. DZH and I are now firm Fred Olsen / Cruise & Maritime fans.
But, everyone is different. For families, the larger ships are better because they do have the room and facilities to offer children's programmes.
It really depends on what you're looking for.