Assassins get +3D6 SA and two different forms of instakill - you're right, they totally suck.
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Helplessness, inc. Stunning Blow, no longer provides auto-crits; instead you get +50% dmg to them. So it's still useful and fun to Stun enemies, but it's no longer the instawin it once was. [And it meant the end of heavy picks' time in the sun in epics.]
A tactics-oriented, possibly WIS-based pure light monk might fit the bill. Highest base run speed in DDO (+30% @ lvl 18), various stuns like Stunning Fist & Kukan-Do, good AC, some healing & buffing capabilities from light finishers, and Tumble as a class skill if you really wanna do backflips.
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I guess the main drawback gear-wise is being limited to robes & handwraps; can make gearing one up a PITA compared to, say, a pure barb who just grabs whatever's the biggest 2H weapon he can find.