I am expressly not listing any elements involving the validity of the allegations themselves, but only such ones that may influence Assange that are outside the scope of the alleged crimes. There is more to be said (see links in the text), but here are the main points of swedish behaviour which are likely to trouble Assange:

• Preliminary investigation: failures to follow procedure and the probable biais of an investigating officer due to personal acquaintance with one of the defendants unfortunately put the investigation itself in a bad light.
• No case: the investigation was opened, closed, then re-opened on request of the lawyer representing the two women. The investigators themselves did not consider the evidence sufficient for court.
• Proportionality: the fact that Assange has already been held in house arrest for over 500 days and faces at least short imprisonment in Sweden prior to any trial raises serious proportionality concerns. Given this result, I believe the argument of Assange's lawyers in court, that the use of an European Arrest Warrant simply in order to further an investigation before any actual prosecution takes place is disproportionate, to be valid. However, opionions are divided on this issue.
• Questioning in the UK: there is no legal problem whatsoever for the prosecutor to do this outside of Sweden, and indeed Sweden has questioned defendants or suspects abroad in the past. This is a decision that is entirely up to the prosecutor, and that decision has not satisfactorily been explained (audio interview with Swedish prosecution authority). Only recently, one justification has been given, and it is essentially one of prestige, which is a rather poor explanation given the fears expressed by Assange and the lack of any non-extradition guarantees by the Swedish government. I would be very curious to know what the real justification was before the case blew up.
• Connections: as mentioned in previous post.
• Expected future behaviour: opinions expressed in the press by Swedish journalists and intellectuals have become virulent, to say the least. Along with the media, public opinion has swayed in disfavor of Assange, giving cause for Assange to fear a fair trial, should it ever come to one. This case would likely collapse before any trial, according to Ove Bring, professor Emeritus of the Swedish national defence college (see previous link). However, with media and public opinion and concerns of prestige being as they are, this has become less likely. Interestingly, similar reasons may be given to argue that an actual extradition from Sweden to the US has become even less probable. It would go down very poorly in the court of global, public opinion should Sweden do so after months of trying to get him to return to Sweden for completely unrelated reasons. Personally, I believe Assange's fears have alway also centered around receiving a fair trial. With time, this fear has probably become increasingly real.

The reasons for him fearing extradition from Sweden but not from the UK has little to do with the respective legislation of the two countries.

The reasons he fears extradition to the US from Sweden are far more likely to be found in the extraordinary behaviour of the Swedish authorities throughout the evolution of this story (well documented), as well as the connections they seem to have been entertaining. Many governments (among them the US and Sweden) will disregard the rule of law if they think (or want the public to think) that the gravity of a given issue warrants it.

Perhaps such fears are unfounded, but given the circumstances, I understand why he entertains them. I probably would to, if the loss of my personal freedom and safety were at stake.