A large chunk of this is I think that as a country the UK is better off within the EU - the common market is very useful when buying stuff from the EU, the various bits of harmonisation help with manufacturing, the ECHR is a Good Thing (once you get past people using it for petty things, like complaining that a footpath being temporarily closed is a breach of their human rights), and so on. There's chunks that could do with reform and bits that don't work so well (I don't think the UK should adopt the Euro, but that's another rant), but overall I'm in favour of the EU.
Another part of why I hope we stay is the leave campaign are a bunch of liars. The "£350 million a week" that they claim the UK would save is a lie, as once you take the rebate into account the UK only contributes about £250m a week. Oh, and we get a chunk of that back through subsidies and funding for all sorts of things so the net figure is somewhere around £170m (source). They're also lying about the UK having to fund potential future Euro bailouts (nope, unless the current agreements change), and are also claiming that remaining will lead to untold immigration. Basically, "leave the EU or immigrants will come over and take the jobs that won't exist".
This isn't to say remain have been much better - they've been flinging their own FUD around (with a general theme of "leaving the EU will destroy the economy") but they don't seem to have been anywhere near as nasty as the leave campaigns. This is important as to some extent this referendum will decide who is PM: if leave wins, then David Cameron will likely step down and Boris Johnson is almost certain to be PM. Which to be fair would be hilarious... but best watched from a minimum safe distance. Like Australia.
It reminds me quite a bit of the Scottish independence referendum from a couple of years ago, where the yes group were claiming all sorts of things would happen which blatantly had no chance (equal control of Sterling? Future UK warships built in an independent Scotland? Joining Schengen without adding border controls with the rUK? LOLNOPE), the no group were being much more realistic about the prospects of a split UK, and both sides ratcheted up the FUD as the date approached. My personal view was an independent Scotland would have been significantly worse off while rUK would do alright, and I think it'd be much the same if the UK left the EU - the UK would suffer, and there wouldn't be a significant impact on the EU.
There was one bit of hilarity a colleague spotted the other day: Nigel Farage was on BBC News spouting some nonsense, and the BBC's automatic subtitles flashed up "Nigel far right". Which I think rather sums up the current state of UKIP - after all, they are full of fruitcakes, loonies, and closet racists.