As such you don't tend to get the kind of insurgent pressure from below that every member of Congress has to put up with. It makes for a very different political culture, although obviously it isn't without its drawbacks.
If by insurgent pressure from below you mean the actual voice of the actual constituents of a member of Congress, well, is that a bad thing? Between the voice of the Party consensus, and the voice of the congressman's actual constituents--yes, yes, I'm not taking into account the
donors in this one, so be it--I would pick the latter over the former. As someone who is often at odds with both parties I would rather congressmen listen to their own constituents than the opinions of John Boener/Nancy Pelosi and a few key party men in smoke-filled rooms. I often quote John Adams: "If I could not go to heaven but with a party, I would not go there at all." Despite what Ricky mentioned about the polarization of our system, I would say, yes, it's getting that way but there is still plenty of local variance in American politics (we just do not notice it via the media because they cannot report on the opinions and doings of 535 members of Congress at the same time).
As far as the independence of members of Congress--even these days--from the voice of their party, well, if you have an iPad, there is a cool app called MyCongress. Probably they have it for droid as well I'd imagine. If you look at some of the entries you can see how often that person abstains, votes with the party, the committees they're on and so forth. And of course their office location in case you want to throw flaming bags of dog poo on their doorsteps and email to send them obscene letters.
I have noticed that Senators are the more maverick-y among the two chambers. Logical: senators usually started as congressmen first. It's considered a step up to the Senate, unlike in the UK where I believe you call it "kicking them upstairs" (and of course they are elected by the voters of their states, not appointed by the president, or someone else). As they have been there longer, they have built up an independent power base of their own, and rely less and less on the Party machine. (And do not forget, local party organizations do not nominate congressmen, they are "nominated" by a petition circulated among voters directly; then they are automatically on the primary ballot.)
This is true in the case of Lisa Murkowski. She actually lost the primary to a tea-party faction candidate, but by a successful write-in campaign she won the general election anyway. It's hard to say who really runs the Republican Party these days, but despite what you may think of the polarization (which is true, and believe me, nobody here feels it worse than me as I am gay but have a few conservative to centrist views, still) of state and federal politics, it is not an absolute. Party politics still vary greatly from one state to another. Take into account the recent state GOP platform in Texas which I understand was horribly homophobic; vs. the Chairman of the GOP Central Committee in the State of Delaware, who is himself openly homosexual. (Probably why the Republican "establishment" in that state wanted the re-election of someone called Mike Castle, in lieu of the very strange woman, I forget her name, who is a tea-partier, who won the primary, thus tossing the only Republican seat in the Senate from Delaware into the hands of the Democrats). If you're a Republican in a very liberal state, there's not too much that the national party "establishment" can do to make you conservative...after all, they do want their party to win, right? Better for the party establishment to retain someone who crosses the floor quite a bit, but is still somewhat within the same party, than have someone who is in lockstep with the national party in a state or congressional district where being so will cause them to lose the election. Yes we've polarized within the last 20 years, but not enough to reverse what I just mentioned.
Murkowski, it says, votes with her own party (GOP/Republican Party) 65% of the time. That's not a very "loyal" party member. So much for party control, whoever said that. Of course, there's always the senior senator from Maryland, Barbara "Babs" Mikulski, who votes with the same (Democratic) Party 99% of the time; and the junior senator from Maryland, ditto Mikulski.
Another question: because the executive is part of the legislative in the UK and Canada, where do bills come from? Who usually introduces a bill? MPs who are members of the cabinet/Federal Ministry?
It is my understanding that very few bills come from the White House. The President is empowered, by the Constitution, to "recommend measures" for the Congress' consideration--he can put a proposed bill on the floor of the House & Senate. Where, of course, anything can happen to it after that; there is no guarantee of passage in the "same" form as he suggested it even if his party has a majority in that chamber. The vast majority of bills originate from members themselves.