Personally I find myself solo ranking to master skills until about 20/25, then just ranking as quick as able. Basically doing what I can to have the least amount of time from 25-40. Then I tend to slow up and work on more skills and solo a few levels until it's time to hero.

My biggest reason for going through mid level as a whole quickly is usually cause certain classes are just meat versus others, where as at the hero ranks I think any class can compete with any class. Mid level you get annoying things like level sitting assassins working on assassinate, a-p/s looking for charges and much higher level low xp penalty classes that just shred you to pieces.


Pking:
I can't thing of a better system for pk ranges unfortunately, but fighting people with 9 levels on you can suck. 900/800 xp penalty classes just have a bad time of it in the mid ranks, and likewise they almost never have people of lower rank in their pk, so they don't really have many viable targets early on. I just find the differences between characters is far more pronounced in early mid ranks than it is at hero.

Exploring:
As has been said, people hate to lose exp, and so they don't take major risks for the reward. The only benefit of exploring earlier is enhanced exploration xp, but even at double, the risk is not worth the gain for the most part. I do like to explore and look at things more with a hero, because I can do so far more safely, and if I see a mob with perhaps an interesting item, I may be able to actually get it.

Rp'ing:
Well, that's just for players to work on, and its really a problem at all ranks more so than just mid. Imm incentives work to a degree, but there are too many players for the Imms to give us all constant attention. Low level Imm quests might be a good start, like putting a quest mob to kill in a fairly under visited area that might be difficult to fight unless they group, and maybe a little bit of a puzzle to solve it more so than go to X and kill random mob for 500 immxp.


Just an idea or two:
I think heralds could play a part in helping people get into all these aspects (though PK may be a touch harder), because they tend to be RP driven instead of PK driven. Maybe they can run a few more unofficial quests (not kill tiamat and the like, but something for mid ranks to do- and maybe they give their own rewards for it- like help with ranking/better gear etc.) Maybe ask them for knowledge, like if you can tell me "so and so" then in return you'll get "so and so" And you could tie it into exploration by maybe asking them about an obscure little thing in an area that might be slightly dangerous for them (linking it into the exploration thing)

I used Zannon (a horrible storm warrior) sort of that way, giving people gear if they brought me Flotinot's last cookie. People seemed to have fun looking for it, and really was not dangerous but may have gotten them to start interacting with other people for clues or really exploring a general area. And even a lot of vets had no idea where it was, cause they never had a reason to know. I can think of lots of things that are not really well known that could be used in such a fashion, and I think it can be fun for the hero and the mid level.