As we just reported, Square has grabbed its biggest vendor yet, Starbucks. Not only is this a fantastic financial partnership for Square, but it’s also a one that will shove the brand and alternate payment system into the mainstream.

When I think about payments, fun isn’t the first word that I think of. After all, someone is taking money away from you. At the same time, when you’ve gotten services or goods that you appreciate, you gladly pay and perhaps review the business later.

Tonight’s announcement just proved to me that a de-facto payment solution, such as Square, could turn the entire payments industry on its ear by acquiring a company like foursquare. There’s quite a few reasons for this, so I’ll lay out the three obvious ones.

Reviews and presence built in

While Square’s app does lead to you a venue through its own directory, there’s not much data to have fun with there. If you were to toss in a check-in, mayor, tip, comment or picture, then things start to get really interesting.

All of the information that foursquare has been collecting for years, and now has exposed in its “Explore” tab, is ripe for instant payment and reservation systems. Talk about making your life easier.

Imagine if the loop could now be closed by paying for something, then leaving a review. Quickly.

A better social graph

Having your credit card tied in some way to your social graph. Since foursquare friends aren’t as loose as following someone on Twitter or rigid like friending on Facebook, it has a nice little graph of people that like to actually go places, do things and spend money.

Tie that social graph loosely to your credit card and purchasing intent, and you have a lethal combination.

Sales team and partnerships

While it’s quite clear that Square has no problem making partnerships on its own, foursquare has been at this game for a long time. It’s hard to go somewhere in major cities without there being a venue on foursquare for it. Once you check-in, you can get a treasure-trove of information like who else is there and who the mayor is. Of course, you can also get deals. Wouldn’t it be cool to pay for that deal from the app…with Square?

Foursquare has a killer sales and partnerships team that would be a great match for Square’s team and culture. The nitty gritty and on-the-ground foursquare savvy would mesh well with the clean design-focused Square. If you could get Square reader in the hands of all serious foursquare venues, some fun things could happen.

This is of course just an idea, but one that I’ve thought about for quite a while. Starbucks just happened to shake it loose for me, or probably the caffeine.