They're plot driven, pacy reads, with an emphasis on action, rather than relationships between characters. Reading them is a lot like watching an action thriller. The detail is in the stunts and the gadgets. You normally can't help but like the main character in each one, because they're often orphaned or alone, or separate from others in some way. They are everyman, ordinary characters in lots of ways, but they always have a few understated talents that make them extraordinary.
I've enjoyed the Alex Rider books by Anthony Horowitz and the Cherub series by Robert Muchamore. The Charles Higson series Young Bond looks popular too, as does the Zac Power spy series for younger readers. It's interesting that primarily adult writers have started to write YA series recently. Who has read the new YA series by James Patterson, Chris Ryan and Andy McNab for instance? What do you think of them?