First of all, I have to say that I am incredibly impressed with the amount and quality of responses I got in such a short time! Especially on a what-bike-should-I-get question which I'm sure appears on these forums far too often and repetitively. I wish I hadn't been in meetings all day so that I could have followed the conversation real-time. It's going to take me a little to go back and really read through what people have said so that I can properly respond, but I wanted to first take a minute to thank you all. I really appreciate all of the advice.

A few quick notes based on my skimming:

-The Dahon Speed D7 that I used to ride was a good fit for me size-wise. I'm a 5'8" female.
-The rolling ability does play in, so I'm glad that's getting a lot of attention. My office building is barely bike friendly. Myself and the five other bikers (in a downtown DC high rise!) have been working on it. We managed to get racks put in the parking garage, but I would still rather take my bike into the office with me (especially if it's a folder). Like I said, we're working on it, but wheeling bikes through the lobby is pretty frowned upon. However, the management seems to be OK with the folder when it's all compact (I've gotten them to treat it as though it's a wheelchair or stroller). I think they're worried about taking up space in a busy lobby. And it is busy, which is why I want more control than a wobbly balance on one wheel.
-Weight isn't super important, but a lighter bike would be nicer, especially since I live on a second floor walkup.
-Cost is absolutely an issue, I do work at a nonprofit. However, I was interested in people's reasons for switching between the brands and models, because if it's worth it, I'm willing to scrimp in other areas to make an investment in something I use every day. And of course there are used bikes.
-The bike will mainly be used just for commuting. I typically bike the whole way (5 miles each way), but occasionally mix-modal commute with the bus or subway. I have another bike that I can use for longer rides on weekends.

I'm looking forward to reading through the comments in more depth. Thank you all, again, for taking the time to weigh in on this. I really appreciate it.