Well... First, remove the add for your notification plugin that is plastered on every page of my WordPress. I could be Ok with it being on the main page for the plugin settings, but not on everything. I really don't need or want that notification plugin as WordPress already has update notifications built into it....so I personally just don't see the point of the plugin really.
Then, don't just have whole sections that can't even be used. Like the coupons section for instance. If you aren't at least going to give us limited use so we can get a feel of what the upgrade would afford us, then there is no point to even having that section there. IMHO, if you really want users to see the benefit of the pro version of anything, you have to give them a taste or sampling. For the coupons section, give a limited number...like 5.
As far as the whole user experience... When I design something I try to keep two things in mind.... Simple and Stupid. I try to keep the look and feel of the website, graphic or mobile app simple and not too complicated. I then try to keep stupid people in mind. And I don't mean that as an insult to users. I mean that I try to keep things very easy for anyone to use. Face it, there are a lot of people out there who barely know what the internet even is or who barely even know how to use that nice, expensive new smartphone they have. So I try to keep the over visual experience simple and easy to use/navigate. Honestly, if the devs who make WP-eCommerce would keep it up to date, I'd probably still be using it. Take a look at that plugin and how polished it is and how easy it is to use even for "stupid people". It also doesn't hurt them that their plugin is fully featured out of the box for free. That's something you might want to keep in mind should they get their house in order and start updating the plugin. They are giving users a fully featured plugin for free compared to the $40 your pro version costs (or you could get your pro version for free if someone wanted to pay a crazy monthly fee for other things too).
Ideally for me, aside from desiring a simple, clean, fully featured eCommerce plugin, it would be nice to have the same with an invoicing plugin. Someone should create a fully featured plugin for freelancers, etc. One that packs in all of the features we want/need....eCommerce, site tracking, ftp, invoicing, newsletters, etc. Like a freelancers plugin suite or something. Now that would be a plugin worth paying for. A freelancer could do everything they need from right there in WordPress and they could do so anywhere they are.