Essentially, in DC a scooter = motorcycle. So the same laws and rules apply. If you are a Maryland resident I think the rules are almost the same except for possibly the 50cc bikes (which I do NOT recommend buying – not enough power to deal with traffic in the city). In DC, you need license and registration for all scooters. I believe technically you cannot park a scooter on the sidewalk. Although I see it so often I’m not sure that rule is ever enforced, although it may be enforced to some degree in the downtown area. Look for available motorcycle parking as your best bet if you want to be completely legal. I pay for parking downtown because I don’t want my bike out in the elements or stolen.
I live in Petworth, am 51 years old, and just got a scooter this summer for commuting into downtown. I love it. But I bought a BIG scooter – a 400cc Piaggio MP3 – http://www.piaggiousa.com/scooters.html#!s=overview/mp3-400. I’d advise you to look at bikes with some power (probably 250 or above) if you are going to commute from Tacoma Park to Courthouse. You need to be able to move with the traffic and a smaller bike will struggle to do that. I’d recommend talking to the guys at Modern Classics about choosing the right bike. Be choosy! And avoid the cheap Chinese bikes – they are essentially disposable. You’ll see them advertised on craigslist as new for less than $1000. No way.
Regarding splitting lanes, bike lanes, etc., I’m pretty conservative about that. I wouldn’t use a bike lane as a primary route, but I do use them to make a quicker right turn rather than wait in the car backup. I’m super careful that no bikes are coming up behind me before I do so. I’m a safety freak so I don’t split lanes in traffic – I just wait my turn like everyone else. What the scooter does for me is give me the ability to navigate better than in a car when gridlock has ensued at intersections, and just basic fun and joy of the open air and the accelleration.
My bike is a three-wheeler, so the added traction helps in rain. I just throw on my rainsuit and go. Doubt I would do the same in snow though.
So bottom line is go for it, but check out your parking situation, and plan on dealing with the DMV for licensing and registration.