I agree that he wasn't as interesting or fun as Spalko, but when he wanted Indy to find him the grail, and Indy refused he shot Henry to get the damn ball rolling. What did Spalko do? Put a sword at someone's neck? Big deal. Like someone else said on these forums, she should have put a bullet in Mac's brain the moment Indy said, "Drop dead, comrade" to show she meant business. Then at least when Indy helps them find the crate it wouldn't have felt like he was doing so out of charity.
Regarding Belloq not being threatening - that was the whole point. He was basically Indiana Jones but with slightly lesser morals, who was willing to team up with the Nazis as a "necessary evil" to find the ark. It was the scum he associated himself with that provided the menace. I don't remember a Toht in Crystal Skull. In fact, none of the bad guys in Crystal Skull were really sadists in the vein of Toht, Mola Ram, and Vogel. Don't get me wrong, I enjoyed Spalko and Dovchenko, but the woman needed some edge to go along with her weirdness. They could have let her skewer one person, is all I'm saying.