Just up front you can get cablemodem from Comcast without any contracts, promotions, or any additional services starting around $40/month. I pay $70/month after taxes for 15mbit download 3mbit upload. Additionally, now a days you can get DSL without having phone service, but this wasn't possible 5ish years ago.
Andrew Beck identified the (by far) #1 problem already: it costs a shitload of money to lay all the wiring to allow a neighborhood high-speed internet access, and the companies that invest this money to build a network from their datacenters to the last-mile of someone's residential (or even commercial) property do not want to share their network.
Phone companies got forced to in the 90s as DSL happened. They were classified as Incumbent Local Exchange Carriers (ILECs) -- implying they are the big boys who laid the wiring. They were forced, by the government, to lease this wiring to Competitive Local Exchange Carriers (CLECs) and they fought it the entire way. The biggest CLEC you may have heard about locally is Covad, a company that's sole purpose (when I worked intimately with them in an ISP context) was to lease ILEC equipment and resell it to ISPs like Speakeasy.
This mostly worked out but believe me, sometimes working with your CLEC was a hassle because you were just an ISP, and sometimes getting the CLEC to kick some ILEC ass was a hassle, because they are just a CLEC. It was about what you could expect from the forced-sharing of a monopoly.
As this was happening Comcast came along and built a better network using better technology and probably stated very early that one of their major goals is to not find themselves in the position phone companies did, and I'm sure they are very well protected against an ILEC/CLEC situation and will likely never open their network -- by force or choice. I'd like to throw out that Comcast is built on local (albeit very large) franchises, so it's not like they are completely closing the door to people who want to do business with them.
So now we live in a world where we can go to the monopoly for a reliably fast connection for a relatively decent price. Or you can go the DSL route and end up with a monopolistic ILEC like Qwest (who is a shitty ISP by most accounts, go read the reviews) or a much less shitty CLEC -- which is likely going to be a pretty cool company (like Speakeasy) that charges way too much because they can't compete and make a profit.
Go price Speakeasy's DSL and tell me any of that pricing looks attractive, but that's your best alternative to Comcast in Seattle. And remember Speakeasy was bought from its local owners by Best Buy maybe 3-4 years ago, but they seem to have not fucked it up too badly.
Another interesting thing is happening in some places. Instead of letting companies build these very impressive, datacenter to last-mile, ISP-orientated networks and then lock everyone out like Comcast did, Verizon is doing with FiOS, and the phone companies tried to do in the 90s, certain municipalities are deciding to invest in their own fiber network as more of a city service that can be resold to anyone who wants to buy it. This is happening in a very limited and commercial-orientated way in Pioneer Square right now as part of McGinn's mayoral platform.
But I think a better example is the UTOPIA project in Utah. It is a perfect case study of someone doing exactly what you described: building a modern, sustainable network that cut out ILECs, CLECs, and monopolies like Comcast, and then offering free reign over access to its gigabits of speed.
Without getting into a detailed history, it kind of worked. It was way more expensive than anyone thought so the cities that invested in it lost a ton of money at first, and all of the local ISPs (ran by nerds with a vision) ended up going out of business, but now that it is in markets in Wasatch Valley that actually care about high speed fiber (aka not the far-off suburbs where people give less fucks about the internet), it is picking up, and any respectable ISP (locally owned, ran-by-nerds who love open computing, competent) is now reaping the benefits because they suddenly have a mature UTOPIA network to sell product on, and not very many competitors, and price points that destroy Comcast at the moment. You can get 100mbit/100mbit for around 40 bucks in Murray, Utah, and I hope Murray is seeing some profit too. It's a win-win-win for communities, for businesses, and for open-computing -- but it was a total disaster for the first 80% of its existence.
I hope that's where we're headed. I hope cities are realizing that network infrastructure is as important as electrical service, and I hope they realize all the ways they can monetize their investment in building fiber networks in their neighborhoods, and then reinvest that back into the community.
Sounds a little bit like a hippie pipe dream but it also sounds better than Comcast :)
Anyway, that's large parts of why you are forced into 4 decisions essentially when picking an ISP: 1) wireless like Clear, don't bother, they suck -- wifi at this scale isn't good enough yet (throw all the "4G" offerings from cell phone providers into this category as well). 2) DSL from Qwest, cheap as hell, shitty quality. 3) DSL from a CLEC like Covad or Speakeasy, expensive, significantly slower than cable (but still totally fine IMO), and high quality networking (latency and stability). or 4) Comcast.
FINALLY, for the record, cable is usually a better overall decision than DSL for home internet access, but this isn't always the case, and I have nothing but love for a high quality SDSL line. People tend to throw DSL under the bus too quickly.