DH: It seems weird to have two different rules, but there's something unique (and quirky, in a good sense) about having to manage and play in different styles. If they maintain the current difference, I do wish they would flip-flop the pattern, however. Use AL rules in the NL park and vice versa. That would give NL fans the chance to see the Edgar Martinezes and Nelson Cruzes of the world and give AL fans the chance to see the supposed intricacies of the NL game.
Pitch clock: From what I've heard at the minor league level, this seems to have worked with comparably little effort. It would be interesting to see the breakdowns of guys who have pitched in the minors since that rule started. Have they continued to pitch more quickly when they get to the majors, or do they revert to prior patterns? But I'm for this rule change.
Mound visits: Having six didn't seem to mess with the game much. If the desire was to shorten things, the next logical step is to try five, it seems.
Roster size: Currently, it seems like the dilemma in roster construction, at least for AL teams, is whether to go with 12 or 13 pitchers. With a change, my hunch is that in the short term, most teams would settle on a 13/13 balance. Who knows how the game will change over time, however. For September, I get the arguments. It's nice to give guys opportunity for September callups, but making big changes to the roster during the pennant race doesn't seem right, either. I have a friend who's a AAAA player, and it's clearly been a reward for him to get a couple September call-ups based on a job well done.
Anti-tanking: Uh, there's always going to be a way to try beating the system, and I'm not sure what they can do.
Three-batter minimum: One of the things I worry about with significant changes are the unintended consequences. The 10-day DL, for example, was designed to let teams put guys on the DL a little more easily with minor injuries, but it's turned into a tool for roster manipulation. On the surface, a three-batter minimum might make some sense, but who knows how it would play out in actuality. And unfortunately, there's not a good way to test it, since the minor leagues seem to rarely do one-batter outings.