Hi Ben
Only been into DSLRs myself for around 4 years but my experience to date involves a few different lenses.
Photography is still only a hobby so I can't/won't justify spending £1000s of pounds on gear for something specific.
I started off with a Sigma 18-200mm which was only around £200. Gave me real flexibility as I pretty much shoot anything and everything from landscapes to family parties etc. The trade off with a lens like that is image quality, it's not terrible but there's better
I then wanted to take shots at rugby & cricket matches and the 200mm wasn't long enough to get close to the action so I bought a Sigma 170-500mm. It wasa big heavy lump but did the job OK. Image quality could have been better (again) and the maximum aperture at 500mm was f6.5 which meant it was pretty lousy if there wasn't a lot of light available.
Last year I upgraded from a Canon 400D to a 7D and replaced the 18mm-200mm with a Canon 15-85mm and the 170-500mm with a Canon 100-400mm L.
The difference is like night and day!!! Even my other half can tell the difference in image quality
OK, I have less range than the previous lenses but the quality is better which allows me to crop when needed.
I also have a Canon 50mm f1.8 which is a cracking little lens for around £80. Can get you out of a hole when there's low light and you can't use a flash and will also let you play with very shallow depth of field.
As Tom says, you can't say "You need this lens for this type of shot" and as Robert said, there's a whole World of settings to play/experiment with too!
Before deciding on my new lens for "general" shooting, I used a program which analyses the focal length of the photos in you collection (sorry, can't remember the name of it off hand but Lightroom does this too) that way you can be quite confident of getting the right lens(es)
