Keg beers are generally sterile filtered and pasteurised as part of the brewing process. This kills the yeast, preventing any further conditioning, and the beer is then racked into sealed, gas-pressurised kegs. Such beers generally taste blander than their cask-conditioned counterparts, and the use of flash-chillers or cold rooms (*very* cold!) is standard as part of the serving process. That said, some microbrewers rack cask beer into kegs - though these are usually served with extraneous gas.
In many common brands of keg beer, cheap ingredients ('adjuncts') such as rice or maize are mixed with the malt to cut costs, but resulting in a 'light' beer with hardly any aroma or flavour. Chilling and the absorbtion of extraneous gas jointly mask the lack of flavour - with carbon dioxide you get an unnaturally fizzy pint; with nitrogen (or mixed gas with a larger nitrogen ratio) you get a pint with an unnaturally smooth and creamy head - either way these beers are always refreshing but usually taste of very little. Micro-breweries generally avoid the use of cheap adjuncts, so their keg products usually taste far superior to the nationally available brands. Also, all beers imported from Germany are required by that country's laws to be free of non-traditional ingredients.
I'm not criticizing all keg beers, simply outlining the often little-known qualities of real ale by comparison. There are many really tasty ales which are 'keg' (but plenty more which aren't tasty!), though well-kept cask versions of the same brands would undoubtedly be found to be even more flavoursome if compared side-by-side
