The original question about whether crowdfunding would be taboo seems to have been answered. It sounds like it wouldn't be. I know I think that crowdfunding is perfectly legitimate and compatible with Xenforo's culture of free and paid add-ons.
The problem with crowdfunding is that you will probably get
free riders. Economists generally consider free riders to lead to inefficiency because the payment received by the programmer/provider is less than the benefit that people derive from the product, an idea that can be expressed more precisely using math (briefly, social benefit > revenue = producer benefit = producer cost = social cost. When social benefit > social cost, that means that there is underprovision of the good). Of course, some providers do it because they enjoy it. Those people are awesome, but there aren't enough of them. (Yes, if you're going to hassle me for sounding like a college econ instructor, you're right. That's my job. And I think that it's hilarious that when I went to get the Wikipedia link for free riders I was presented with a javascript popup saying that "If we all gave $3, the fundraiser would be over in an hour." That's the definition of a free rider problem right there.)
Economists would call the pricing model that you are using right now
second degree price discrimination. While most people get the product for free, people who have a high willingness to pay (like many people in this thread) end up paying for branding removal, giving you some cash. It's pretty clear that this is one of the best systems. While price discrimination sounds like a bad thing, there are mathematical proofs to show that it generally leads to an increase in social welfare because more people who want the good end up getting the good. Loosely speaking, price discrimination means that you get more money and we get more software, so everyone wins. Like I said, a good system.
From an economics perspective, the inefficiency might be in the idea of free upgrades. If upgrades are free, then upgrades will be under-provided because, as above, the payment received by the programmer is less than the benefit that people derive from the product ( social benefit > 0 = revenue = producer benefit = social cost, so again we have inefficiency). Given this argument, I'm always happy to pay for upgrades, but I know others don't always feel the same way and, econ aside, it's what your customer's feel that matters.
By the way, if you're wondering why I'm writing all of this, I guess I just hope that you find it interesting. I don't imagine that you'll find it terribly helpful, as I'd bet that you know most of it already, just without the terminology. I don't know if the econ adds that much, but when I apply for a job I tell them that I have a passion for teaching and maybe this means that I'm telling the truth.
Finally, if you'll indulge me a bit, what we are discussing here could be called "market design." I think that's part of what got me interested in this thread in the first place. The leading researcher in market design, Al Roth, just got the Nobel Prize for essentially founding the field of market design starting with when he analyzed the NRMP, the market that matches medical residents up with hospitals in the US. (
Nobel prize link and a
cutting edge article). I actually had the chance to pitch an idea of mine about market design to him a couple of years back. Within 2 minutes of my opening my mouth, he had traced my idea back to top trading cycles, an idea published 40 years ago. Basically, my idea was redundant and it only took him 2 minutes to figure it out. It was humbling.
By the way, setting aside my role as a dismal scientist, I'm not saying that crowdfunding is a nonstarter. You may get some free riders, but you may also have a lot of fun and raise some cash. It certainly seems to be working for the XenTag #hashtag upgrade (
link1 link2) that @
Adam Howard masterminded, but I wouldn't rule out that @
xfrocks just rocks and is doing it because he seems like a really nice guy who loves coding projects. It might also a good way for people to prepay and show their appreciation for well received software. If you did make this a paid upgrade, perhaps you could include the upgrade as a perk to crowdfunders. Perks like that seem to go over well.