IMHO, DoF was a shocking expac for grinding - and - I have found things won't get better till your able to get into SoS/KoS (low 60s)

To overcome the '50s flat-spot' I shoot back and start doing all the grey quests - a simple path to follow will be:
- td (quests 10+)
- darklight (quests 10+)
- greater fay (quests 10+)
- bbm
- el
- feerrott
- steam
- ef
- ls

That will definately get you over the hump and pretty close to the 60s for returning to grinding in SoS - when you get bored of there, hit up the KoS instances for names and disco - then you should be getting pretty close to thinking about RoK

If you find yourself short still - the next best zones to grab a few levels (but via questing) are:
- less fay
- loping
- bone

IMHO, if I missed a zone in the above lists it's because I have either done them to death already or they just simply suck

Of course, if you missed a dungeon, go back and grab disco and any names if they have colour

n0x