I've been using the beta. Seems the only thing you'll be charged for is custom content provided by other companies, like Packs (themed background/display pics/emoticons, etc), premium games and that kind of thing. If you don't want to pay money, just don't buy that stuff.
Otherwise, I've read numerous times that both Messenger clients are free for general use, and that this won't change.
And it's really simple to make it "small", kinda like v5.0. You can turn off the display pics from the conversation window, as well as both of the toolbars. And just about every feature can be turned off, so if you don't like emoticons, winks, nudges, automatic background sharing, etc...just turn it off.
Personally, I like some of the new features. The personal message is a nice touch, and can also switch to show your friends what you're currently listening to in WMP or iTunes. Contact cards are really cool, and I can immediately see when a friend updates their blog and go straight there with one click. I can now sign in as invisible. This is a good one: if I get kicked off the server for some reason, and Messenger tries to automatically log me back on, it uses my last-known online status...in previous versions, I would just be shown as "online" when this happened, and what a pain that was...