The blog
Separated By A Common Language is THE place to go if you want to ponder Britspeak/Amerispeak (and a bit of Oz, Kiwi, Canadian, and others thrown in). Random discussions on accent, spelling, vocabulary, and subtle distinctions of usage. The blogger is an American transplanted to Sussex, where she's a lecturer in linguistics and English, so she knows what she's talking about.
A major difference between UK and US speech is the intonation (the "melody" of speech). Generally speaking, British English has more complex rise-and-fall patterns, steeper changes of intonation, and a wider range of intonation than American. British is spoken faster as well, on average. This makes British English sound more "lively" to some hearers.
The other main difference between US and UK speech is the pronunciation of vowels, but that's too complex to go into here.
This website has a good collection of accents from English speakers all over the world.