One thing that will up Terraria's replaybility for me is more stuff like the dungeon. _Not_ an entire biome, but single smaller structures underground, like temples, graveyards ect.... Digging by far takes the most time in this game, and if they littered the underground with tiny sites, the game would sort of DF like in the fact you play far longer than the powerscale simply because want to see more stuff
In Terraria, the power scale and content are directly linked right now; there's no real optional areas or additional stuff. That will come with time I'm sure...but they definitely should be looking at that kind of expanded content that isn't falling cleanly into the progression of "Surface, underground, deep underground, corruption, hell, dungeon --> islands, jungle."
I've still got stuff to do SP, worlds to make pretty and stuff to build underground. I'm just hoping the content updates keep pace. I have absolutely no problem with making a lot of new characters and starting over to see new content updates. And despite how nice it is to have a character that goes to any world, I think it's actually a design decision they should have avoided. The worlds are the real characters in this game.
On PvP, I wouldn't get too worked up making rules just yet. I think they realize they need CTF as one of the first updates so PvP has a point and has some structure, and it will be coming soon. Ish.