All of the top-down approaches just look ugly to me. You're either looking at the top of people heads, or tokens of disembodied heads. This also gives it a pseudo-3d look, with a bit of clutter to add that homey tabletop atmosphere.
I came to the opposite conclusion. When you add perspective to game visualizations, it becomes harder to judge distances, estimate angles, make out details, and identify borders. Small items tend to get reduced to squint-inducing shapes. Large items occlude each other and cause an infuriating visibility problem. The only advantage is that it looks a little bit more like a "real" world, but if you're using a simplified art style, it doesn't matter anyway.
From a UX standpoint, the best balance is "iconic" perspective - most things rendered flat with low occlusion, like an Egyptian painting, much of medieval art, or Japanese woodblock prints. All the boundaries can remain clearly delineated with this style, but aesthetics can be retained through careful composition of each icon.
I did exactly this style for the game I'm working on. My reference points(within video games) are Ultima 1-5, Heroes of Might and Magic, and Battle for Wesnoth. In these games you are never wondering what something is, because you can always make out what it looks like.
Normally I'd agree with you, for those exact reasons, and my first prototypes were all generally flat (also being a fan of the Ultima series.)
But when I tried the current isometric style on a whim, it managed to solve all of the problems I was trying to address. Probably because I use a hybrid approach: only the underlying battlemat tiles are isometric. The miniatures are flat, slightly-transparent stand-ups and always face the player.
I use Cancel buttons for pop-up forms all the time. It usually just hides the containing div, so re-opening the form will show the data as it was entered previously. Making the Submit button 2-3x longer than the Cancel button helps mitigate the risk of accidentally hitting the wrong thing.
There were even fewer options when I wanted to go this route 2 years ago, so I just went with a standard payment processor (BMT Micro) and charge one-time payments for users to pre-pay for time. e.g. $19.99 for 3 months or $59.99 for a year.
If you want to get from zero to charging ASAP, this might be worth considering. You could probably be integrated in a day or so. They are mainly geared toward downloadable software vendors, but I just use the "serial code" field as an Activation Code that the user enters into their profile page.
BMT Micro's royalty rate is around 10%, have awesome customer service, and just added a recurring option. The main downside is limited control over the template. I would also look into FastSpring since they are newer and seem to have more flexibility with the store configuration.
I have users that spend nearly all of their free time on my site. One recently asked me to ban them so they could kick the habit. I asked him what part of the site was addictive and he said, "Basically everything."
One of the key things, I think, is having lots of little things to do that you can get immediate feedback from. e.g. Check the forum for new replies, rate new content, see what kind of response their content is getting, etc.
Having strict moderation keeps the general quality high, which means it's usually worth checking back to see what's new. There are also multiple tracks of achievement, like hitting 500 comments, etc. so there is always a new badge within striking distance.
You don't even need to have permanent badges. I have a list on the front page of the people who have submitted the most ratings that day. You don't get anything from being on the list, but people try to get to the top anyway. People like seeing their own names.
Having a friendly community is also a big help, since it means that other users are a direct source of positive feedback. That's a bit harder to develop, though.