Given the arithmetic, the Lib Dems are right to talk to the Conservatives first. But the Conservatives have no recent experience of coalitions, apart from John Major's attempts to preserve his majority in the mid-1990s. Labour and the Lib Dems have been in coalition in Scotland and Wales, and both Scotland and Wales currently have minority governments, but the Conservatives have been in opposition throughout the existence of the devolved legislatures.
Moreover, Cameron has demonstrated that he himself is particularly bad at negotiating coalition deals. In my last post I described how his alliance with the UUP has resulted in the electoral wipeout of both parties in Northern Ireland. From the Brussels end, I have watched gobsmacked as the Tories marginalised themselves in the European Parliament by finding peculiar fringe allies rather than remaining a key faction in the largest parliamentary group. In both cases, the impetus for the deal came directly and personally from Cameron himself. In both cases, opponents of the strategy (Sylvia Hermon, Edward MacMillan-Scott) were marginalised and ignored - with the result that both left and did better outside the tent than in. I see a pattern here: that Cameron chooses his allies on instinct, without much strategic depth to his considerations; though in fairness he also seems to stick to his guns. (Another symptom of this is his loyalty to colleagues even if they are not really up to it; I haven't been following British politics closely enough, but this may explain why the likes of George Osborne and Owen Patterson have not yet been relegated.) I deduce from this that even if Cameron strikes a coalition deal, he probably does not have the political skills to implement it in the medium to long term. As I said in a slightly different context above, this means that his partners need to extract their deliverables early in the relationship (as the UUP and Poles did, in terms of getting money and parliamentary chairmanships respectively), since there probably won't be a later.
Not that Gordon Brown impresses me as a potential political partner either; I've read Paddy Ashdown's account of Brown's rather poisonous contribution to the Scottish coalition negotiations of 1999, and his awful behaviour even to friends and allies, let alone potential rivals, is legendary. A different Labour leader might be a different matter, but I don't think that the timeline allows for that option. (In any case, the numbers do not really work for a Lib/Lab arrangement.)
Clegg does have experience of coalition-building from his time in Europe, probably more than Brown and certainly more than Cameron. Probably also more than most of his own party, many of whom instinctively regard both Labour and Tories as evil and have difficulty in believing that either could be a necessary evil. I quite like Clegg, but his own internal authority has surely been tarnished by a bad election result. I certainly don't envy him.