The Voter Power website is good at illustrating some of the problems with the British electoral system. Essentially, votes in small marginal constituencies count for far more than in large safe ones. (HT Bishop Alan)
Another issue is the whole clash between constituencies and parties. Realistically, most people vote for the party they want to win, but who they elect is a local representative who may or may not have anything to do with the area and may or may not be competent. When there is only a fairly small parliamentary majority (as in the Major government), the identity of the MPs matters quite a lot, but is largely irrelevant to the electoral process.
Furthermore, the Prime Minister is elected on a very small set of issues, but then proceeds to implement their policy across a whole range of issues. Topics like abortion and the death penalty are strongly emotive, but we don't get an opportunity to choose either way.
Most of the time, the British system works like an elected 5-year dictatorship, with decent representation for the communities who voted for the dictator (because those MPs are in the government), and less good representation for those who didn't (because those MPs are in the opposition). Of course, the communities don't have much say in which individual represents them, just which party. And that seems kind of dumb.