In general, international flights to the US remain remarkably restricted and regulated. Foreign carriers aren't allowed to transport passengers within the country, transit without a visa is completely impossible, etc. Compare this to the EU and (soon) even South-East Asia, where anybody can fly anywhere they wish, as long as they fulfill standard safety requirements.
On the flip side, things for travelers might also get worse, or even more "low costy", if we had domestic fifth-freedom flights. In order to compete with European low-cost carriers, Lufthansa migrated a lot of their intra-Europe flights to low-cost affiliate Germanwings, whose travelers don't get the same perks or lounge access as people on normal LH flights. This has been kinda a pain.
But competition can be good. One nice perk of the Norwegian Air incursion into the transatlantic market has been the ultra-cheap Star Alliance retaliation fares to Northern Europe airports that DY serves. There are some amazing $500–$600 round trip fares from either coast to OSL, and you can turn them into a trip to "anywhere in Europe" by using stopovers wisely.
This article mentions that the Azores are another potential hub for low-cost carriers but doesn't mention that SATA (S4) really does publish some dirt cheap fares to Europe, BOS-PDL-LIS at like $300ish round trip, albeit on a sketchy A310 aircraft with low-cost service.
Is there a good website to find deals like that?
When you go to buy the ticket, the price may not match, so be aware of that. I think it's usually right, though. I bought a similar flight at a similar price, so I can sort of vouch that it's not some garbage quote. Google Flights is pretty good for this, at least if you're willing to tinker with the dates and cities until you find the magic spot. They have a little bar chart of the prices, so you can look at surrounding dates and find better deals. I don't know if I'm just doing it wrong, but the other flight search sites I've tried sometimes can't find the deals on Google, even when I search specifically.
Also, consider booking on www.norwegian.no for probably ~$30 savings, but it's only in Norwegian, so if you can't read it confidently, maybe that's a little too crazy.
United has a live $69 base fare K fare from EWR-OSL; find inventory to match and you're good to go. ITA Matrix (matrix.itasoftware.com) will help you do a fairly broad search, like "departure date within 1 month of X day and total journey length Y--Z days."
If you don't have the time for that stuff, follow the folks at theflightdeal.com, who as near as I can tell have software scraping fares on a few hundred key routes and make blog posts when they see particularly low fares; see e.g. http://www.theflightdeal.com/2014/10/31/united-466-newark-os... .
The SFO-OSL $512 fare was posted in March; don't think it's still valid, http://www.theflightdeal.com/2014/03/07/united-512-san-franc... .
I flew to Oslo earlier this week from Glasgow (via London Heathrow), on British Airways. I got:
* Hold luggage.
* A sandwich on the ~2h flight from LHR–OSL (and a biscuit on the ~1:30 flight from GLA–LHR).
* Tea and a soft drink on both flights.
This is the sort of level of service I'm used to being included in the ticket price on relatively short flights — to find no major American carrier does this amazed me. It's also worthwhile noting that compared with low cost airlines like EasyJet and Norwegian, British Airways is typically around the same price in my experience once you add-on the above to the base fare of the others. (RyanAir is cheaper, but does so by strong-arming minor airports into charging them next to nothing, which results in them flying to relatively distant airports.)
Or, United not carrying amaretto on some flights, or having the nerve to charge $6 a drink for it in the lounges.'
Spirit's even worse.
 http://www.bluelagoon.com/ - no, not a natural geothermal spring, but no less relaxing for it.
I just tried a couple from a quick search but wasn't impressed with their interfaces. Anyone care to recommend a good online tool with this functionality?
http://www.gcmap.com/mapui?P=BER-KEF-BOS;NRT-HEL-CDG&MS=wls&... (Berlin to Boston via Iceland; Tokyo to Paris via Helsinki)