There's a lot of fuss right now on Twitter about the agreement to only allow a dissolution vote if 55% of MPs vote to do so. With the Tories likely to hold over 47% of the MPs it becomes impossible for parliament to vote for a dissolution of parliament on the fly.
Does this mean we have no choice but to have a Cameron government for 5 years? Not at all. No confidence votes will still stand, including on issues such as voting down a budget. If 50%+1 MPs vote against the government on this then the result is either the resignation of government or the dissolution of parliament. Even assuming that (quite unlikely as it is) Cameron will force it so that you cannot dissolve parliament this way, it still means he'll have to resign as Prime Minister and the Queen will have to ask someone else to form a government.
The end game of the above scenario is, surely, the Tories then agreeing to a dissolution vote or face the ignominy of being the party that dragged the country through instability for the sake of spite. Again, this assumes a no confidence vote wouldn't, as it currently does, practically equate to an election being called.
The 55% is a safety barrier, it stops the Lib Dems from getting their AV system, cutting ties and working with other parties to call and election and profit from it. Given it is the Lib Dems that are most likely to break away from a coalition it is a practical step to ensure that government is only compromised in true issues of no confidence, to maintain the integrity of the idea of a fixed term parliament.
Edit2: It also, as I should have said, stops the current largest party from forcing an election at an opportune time under a fixed parliament too, the point of fixed term parliaments being to try to keep governance running until it can no longer do so. Some have suggested that this should be a referendum issue too. I should be clear that I am personally not sold on fixed term parliaments, but they are pointless without this kind of threshold rule.
Edit3: Some feel this is all undemocratic and without precedent. Scotland operates fixed term parliaments, and their threshold for dissolution is 66%, higher than 55%. The reason for this is because fixed term parliaments are intended to keep on going, if a coalition fails the first course of action should NOT, under a fixed term parliament, be an election...it should be giving another coalition or minority government the chance to rule. They also have a 28 day release, which means if no-one is able to gain power to govern, to protect against the sort of thing I state above about keeping a parliament crippled, an election is automatically called. I'd fully expect that to be the case for the UK as well, though we have to be calm and wait for the full details.
Edit: For more on confidence motions, this parliamentary resource seems quite good.