[PhasmaInMachina]

It sounds largely like an issue of game balance. I think some MUD designers are certainly guilty of making "badass" type races and classes disproportionately powerful.

The thief class you described sounds something like what we have. We have four sub-classes of thief (though you can mix and match the skills.) The first is a straight-up pickpocket making them very difficult to pin down even for a fighter, and they are capable of stealing stuff pretty easily but their melee is poor. The second and third mix poisons and make traps respectively. The fourth is a binder capable of rendering most opponents helpless if they manage to knock them out and throw the bindings on, and the fifth is a thuggish type thief, built for melee who employs a range of underhand tactics in combat to disadvantage unsuspecting enemies.

None of the thief sub-classes have the up-front edge over a melee class (like a warrior, or a paladin), but they have a series of annoying and/or underhand tactics to employ. Plus stealth.

The assassin class is a fairly strong defensive melee class with a lot of utility in their combat moves. However, once again they will not be able to compete in most cases with a decked out warrior. Assassins get the ability to assassinate (one-shot) any character, but it is a high-risk skill (often ending in the assassin's death if done wrong) and takes a lot of time, luck and preparation on their behalf. It is rarely successful.

Essentially both classes have their own little niches. When it comes to melee combat, very little can go toe-to-toe with a paladin, or a warrior in an up front battle. Stealth classes should be about subterfuge and cunning and shouldn't play like warriors.