I would love my client's web app to take off, but I feel that there is much improvement to be had before it can be completely usable.
What are some resources that will allow me to get feedback and advice/suggestions for improvement/addition of features that will make it more usable?!
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Basically you need to do user testing. That will highlight areas where you need to improve.
You should user-test your app at every stage of development. This will alert you to usability problems earlier, which will often mean less work will be required to fix them.
Test early, test often.
http://www.sensible.com/ should be your first stop.
In case you haven't clicked the link yet, an excerpt from the free chapter
We don’t figure out how things work.
We muddle through.
One of the things that becomes obvious as soon as you do any usability testing—whether you’re testing Web sites, software, or household appliances—is the extent to which people use things all the time without understanding how they work, or with completely wrong-headed ideas about how they work.
...
Why does this happen?
It’s not important to us. For most of us, it doesn’t matter to us whether we understand how things work, as long as we can use them. It’s not for lack of intelligence, but for lack of caring. In the great scheme of things, it’s just not important to us.5
If we find something that works, we stick to it. Once we find something that works—no matter how badly—we tend not to look for a better way. We’ll use a better way if we stumble across one, but we seldom look for one.
It’s always interesting to watch Web designers and developers observe their first usability test. The first time they see a user click on something completely inappropriate, they’re surprised. (For instance, when the user ignores a nice big fat “Software” button in the navigation bar, saying something like, “Well, I’m looking for software, so I guess I’d click here on ‘Cheap Stuff’ because cheap is always good.”) The user may even find what he’s looking for eventually, but by then the people watching don’t know whether to be happy or not.
The second time it happens, they’re yelling “Just click on ‘Software’!” The third time, you can see them thinking: “Why are we even bothering?”
Buy the book. It's fun to read and will help you build better, more usable (and therefore more successful) sites.
The question you have to ask is do you have the right features and they aren't organized correctly preventing them from being usable, or are you missing some key features and with those features your application would be more usable. The easiest thing to do is ask your users.
If you have a base of users then I'd suggest signing up for http://getsatisfaction.com/. You can place a button on each page of your web application and this allows people to submit ideas about that page to you, and aggregate ideas so you can find out what the most common suggestion or desire is.
It allows you to open up communication with your users, and have a dialog with them about what would make them use the application.