Yeah, the 2mile sign is rarely implemented for any exit other than a major one. (ie another interstate).

I think that a few major differences help to catch out UK drivers;
1) The density of exits on a US freeway is far higher than on UK motorways, which means that you see a lot more signs generally. It's easy to start missing them because you switch off.
2) The default positioning of exit warning signs is at 1 mile and 1/4 mile, versus 1 mile and 1/2 mile in the UK. That extra 1/4 mile in the UK can make a lot of difference.
3) In the UK you have 300,200,100 yd countdown markers - it's near impossible to miss your exit. I don't recollect ever seeing these in the US. The exit marker is usually something like a yellow reflective diamond which is positioned so that it would be on your left as you exit.
4) UK has long straight slip roads, which are generally easily visible. They are also marked with green cats eyes so the stand out at night. US Exit Ramps tend to be short with sharp turns, making them easy to miss if your not paying attention during the crucial few seconds.
The use of *any* cats eyes in the US is very dependant on state. New Jersey has a lot, Massachusetts has virtually none. Regardless, I haven't ever seen any as clear as in the UK.

As to the quality of signage in general - in Massachusetts they are poor. I think that's a symptom of other issues.

The US numbers routes and not roads. This is a big problem IMO, since a major road can end up being part of several routes in different places, so the numbering is constantly changing. Taking Boston as an easy example;



That red ring road around Boston is Massachusetts State Road 128 (colloquially as '128') But from where I95 meets from the south to where it leaves at the north, 128 becomes I95. If you look at the exit numbers for 128, they follow I95's numbering clockwise for this portion. Now the southeastern section of 128 is also known as I93, and the exit numbers also reflect this, numbering counterclockwise from 1. The kicker here is that I93 North is 128 South at the same time. Oh, and I nearly forgot to mention that this section is also US Route 1 North as well. Now the northeastern section of 128 presumably contains true 128 exit numbers. Running counterclockwise from 1 at Gloucester (off map) to 28 just before it hits I95.

Confused? Join the club. It seems as though no one seems to know what to call this road at various points. If you're looking to get on it, sometimes you'll see signs for 128, sometimes I95, and sometimes I93, depending on where you are. And often they'll be wrong. And listening to traffic reports is interesting too. The Eyes of the Sky generally refer to the whole road simply as 128. But they use the exit numbers appropriate to the individual routes, so it's up to local knowledge to decipher what they mean when they talk about '128 Exit 20' (Should be on the northeastern section, but they usually mean I95 Exit 20).

And on Cape Cod, there's a road, route 28 that is labelled North for South and vice versa along it's entire length. It's really confusing travelling North with the sun in your eyes. At some point obviously someone got pissed off about it enough to do something - some of the signs have been spray painted with a correction.