He was just some Sith Lord, strong but not all powerful. At least with Malak, you had this connection to him, this personal score to settle over his betrayal of your former self. The villains in general sorta sucked in TSL. Gee whiz, hidden Sith and boy they're dark, and even more evil! And that Nihilus in who's vision I'm no more than a spec! Oh, did I just cleave him down with a handful of lightsaber strikes? All that build up for that? Only I, the exiled Jedi, could've managed that?
Don't get me wrong, some stuff was good/interesting, but how many times did I really need to hear about the force "echoes". I wouldn't really call TSL's writing "great".