Political Oligopolies

It seems to me that many democratic systems have evolved in to political oligopolies where two or three political parties dominate all other competition. This reduces the choices available to citizens. It is especially problematic when you want to vote against a government at the next general election and realise your options are limited and often unpalatable.
The main political parties quite naturally play the democratic game to gain power at every turn. And the media may be complicit in this game, whether by accident or design. For example, the BBC does not give airtime to new politicians that can not demonstrate support (often measured by opinion polls), creating a barrier to entry for any new competition. Commenting on the London Mayoral Elections Lord O’Donnell, who ran the civil service as Sir Gus O'Donnell between 2005 and 2011, said the current rules were “ridiculously skewed towards the status quo” (quoted in The Telegraph).
The branding power of the political parties is especially powerful in the long run. The repetition of party names makes them just as trusted as other commercial brands, and just as powerful. Then we may factor in their ability to raise campaign funds, and how one feeds off the other, and pretty soon the dominant parties become an oligopoly.
Interactive Democracy would break this strangle hold without breaking the parties themselves. It could create a new wave of competition among ideas to the benefit of all, just like competition between companies is often thought to improve the economy as a whole.

The Rise of the Independents

Martin Bell has launched the Independent Network, a kind of "kite mark" of approval for independent candidates which may go some small way to enhancing their presence in the political market place in order to compete with the big brands: Labour, Conservative and Liberal. More here from the BBC.

Should all the candidates pass some sort of exam before they can run to become MPs?

This would potentially undermine representative democracy in which voters are the final arbiters of "good character", but wouldn't necessarily impede Interactive Democracy as voters would have more power.

What would such an exam include?

Statistics? Logic? Parliamentary procedures? IQ?

More here on the qualities of a professional politician.

Political Barriers to Entry

It's interesting to note that that the three biggest parties capitalise on their leading positions by garnering far more air time than any of the minor parties, thus reinforcing their position. This creates a strong barrier to entry for any other party or independent politician trying to get seats in Parliament.

Quite apart from the power of their brands, the leading parties also attract more funding than the lesser parties, which enables them to entrench their power base. They have money to spend on a much more powerful campaign.

Occasionally, independents do get seats in Parliament, but usually they already have some sort of brand presence in the media. For example Martin Bell, with his distinctive white suit, was a BBC correspondent.

These political barriers to entry are another way that democracy is skewed from the ideal. Interactive Democracy would give voters more power, allow new ideas to emerge and foster wider debate of individual policies. However, it still relies on party politics to form a credible government and does little to undermine the three main parties as political brands.

(Please see Porter's 5 Forces as another perspective on the power balance within politics.)