When I look at most of my favorite TV shows I must say they're mostly American. These include Breaking Bad, Dexter and especially Sci-Fi shows like BSG and Firefly. The main reasons seem to be the obviously larger budgets and "level of perfection".
On the other hand, I must give credit to British TV shows especially, because they have their very own flair. But I'm having a hard time pin-pointing what exactly I like about British TV shows compared to American ones.
Here's a few reasons I could come up with:
-British TV has less "unwritten rules" about what's ok and what's not. Yes, children can die and pretty women can get killed as well. Something that rarely happens in U.S shows.
-More realistic characters: To me it seems like British characters come across more realistically and believable than in some american shows. It's not so much about the acting but rather how a character is written. E.g. it's far more believable that a certain British TV show character might exist in real life than it would be for some US show characters. They have a tendency to the extremes.
-More believable plots: U.S. shows tend to be very dramatic and up the stakes to increase the dramatic effect. The world is saves twice a week at least and the villains tend to always strive for the highest possible goals. In U.K. show however, things tend to be kept simpler. This might be partially due to budget constraints but it also adds a lair or "realism". Just compare a Series like Misfits with Heroes for example.
-Story defines the length of a Season: For U.S. shows it seems you have the standard formula of either 13 or 22-24 episodes per season. This is probably due to how the TV show business works in America. However, U.K. shows seem to follow different rules and so a TV season can last anywhere from 4 to 24 episodes. In a way, the plot defines the length of a Season, rather than the industry.
-Humor: While there are exceptions, I've found that even the most serious British TV show will contain a few one liners or quick jokes.