I once spent a very pleasant summer working in a decent wine shop. It was a lovely job that I would recommend to anyone, and I have certainly often toyed with the idea of setting up my own store, if I could find a town large enough and posh enough to do it without having to go head to head with Majestic. Anyway, I thought I would make myself look like a pompous patronising git by sharing with you some of the things I learnt in those halcyon days of talking to friendly alcoholics and having access to unlimited free booze:
Regardless of what newspaper wine writers tell you, supermarkets very rarely have any wine worth drinking at all. Waitrose is an exception. Majestic is very good too.
An empty bottle, a fancy label and a screwtop generally cost the wine producer approximately one pound. The import tax on that bottle is about £2.50. Basically, if you buy any wine under £4, the wine inside is in effect worthless, which means it is very very unlikely to be any good. It is usually thought that the cut off point for a quality wine (made with care) is £6 - of course you can have good wines for less than this, and bad wines for more than this.
Supermarkets are particularly devious with wine offers and promotions. The way they offer wine discounts (half-price!) is to sell wine at double it's RRP. Then they simply halve the price for a week, selling lots as people buy it from the aisle-end displays, little knowing that they are actually buying it at normal full price.
Much of the wine offered by supermarkets and wine clubs is made of blends - created by UK buyers who go to France, Australia or South Africa and simply buy up entire giant metal barrels of surplus low quality wine on the cheap, then blend it together until it tastes drinkable, then stick a whimsical label on it ('The Old Red Tractor') and flog it to gullible UK punters. The label is often created for one blend then disused, which is why the wine club often has all sorts of bottles you've never heard of.
People buy expensive vintage wine and expect it to taste a certain way, and are then often disappointed. Expensive vintage wine or champagne can taste quite strange and can be an acquired taste - sort of like jazz. Just because it's old and expensive doesn't mean you're going to like it (or even that its good - you really need to know about about good vintage years to have a good stab at getting a good old wine)
A high alcohol content has the effect of making a wine taste more 'velvety' and 'smooth' in your mouth.
Chilean reds are probably the best value and most 'reliable' out of cheaper wines, due to Chile's almost perfect wine growing climate. Sicilian whites can be very good for the price too.
Pinot gris is the same grape as pinot grigio (its amazing how many people didn't get that) but it can have a different taste due to the way it's made...One of the amusing things that happened regularly was the amount of people who came into the shop saying they wanted a white wine - "ABC, Anything but chardonnay!(snort, guffaw)" and then they'd proceed to buy a case of chablis, proclaiming how much they loved chablis, of course unaware that chablis IS 100% chardonnay. Same went for people who said they didn't like sauvignon blanc and then proceeded to buy a bottle of sancerre.
*edit: Thought of another one: When you go in a wine shop, they frequently offer you some posh cheese to go with a tasting. This is not as altruistic as you think - the cheese, whilst tasting delicious with the wine, also has the added effect of ironing out many less than perfect aspects of a wine, and can make many wines taste a bit better than they maybe are.
It's a shame that I don't work there anymore, as I certainly never get to drink nice wine these days now that I actually have to pay for it.