Here are a few things Morty has done (from memory):

1. His house is a safe haven from ghosts. He has added ghost dust to the very walls of his home, so that the only point of entry is through open doors—which also require an invitation.
2. His control over spirits is extensive. Morty sent ghosts all throughout Chicago to pull off the map trick—you're talking about a compulsion that spans a very, very great distance. Several miles, at the very least, which is difficult with any magic (though you can argue that the rules for ectomancy are a little squishier than other disciplines).
3. It appears that his abilities were natural, similar to the way Molly can naturally make veils and illusions. But, further than that, he worked at them to get better; Harry states that Morty could very likely join the Council if he wanted to. Essentially, that makes him a black belt.
4. Morty's possession abilities aren't just being a receptacle; he can compel ghosts to hop inside so that he can borrow their abilities, against their will. Not just fighting skills, either; when Dresden popped inside, Harry involuntarily made a shield. That leaves a whole lot of room for magical augmentation; if you can get a wizard ghost to hang out with you, especially one on Harry's level, he could potentially have access to all kinds of things.
5. He was able to rip the Corpsetaker out of Molly's head.
6. He was able to see Butters as a ghost—essentially, just a free-floating sprit—and force him back into his dead or dying body.

Some of the lines he won't cross:
1. He considers the spirits under his care.
2. He does not want to send ghosts into necromantic areas. In Dead Beat, he flatly refuses to get involved, because the necromancers can harm his spirits, or consume them.
3. He has a particular fondness for Sir Stuart, his ancestor, a sort of guardian spirit.
4. He lost his abilities for a while after years of disuse or no use at all. His moral code concerning spirits is, I think, partially related to not wanting to make the same mistakes again (making him something of an Atoner, to use a trope name).
5. He can be charming on camera; Larry Fowler liked him.

Mostly, I like lovable coward characters. Morty is driven by self-preservation, mostly, and typically just wants to be left alone.