Suspension

Full-Suspension mountain bikes have become extremely popular in the last five or so years. Every range has one, and usually several. Most full-bouncers above £300-ish are reasonably well sorted now, but until you get closer to £750 you're likely to get a huge negative trade-off in extra weight and low-spec componentry for your added boing. Consider very carefully whether you actually need it, you'll be a better rider if you learn your stuff on a hardtail, and experienced riders will really notice the difference if you go from a decent hardtail to a heavy, clunky full-suspension machine. You really do get what you pay for, so try it out first.