I personally think there are a number of big sticking points for many people.
1) The fact that no one really knows what is going to happen if we do leave the EU, what will happen to those who are currently here under the free movement, and likewise British ex pats, amnesty?, immediate visas?
2) Immigration, this a huge smokescreen, most of the people who come here from the EU, come here to work, stopping free movement will just stop this labour market, plus it doesn't stop immigration from non-EU countries, that remains the same as it is now.
3) What happens to the laws and rules (the good ones like the working time directive, maternity rules etc) that we've adopted while in the EU?, will they continue, or will they be rescinded, having a conservative government in full control of labour laws makes me twitchy, with thoughts back to the 70's and 80's
4) It's very difficult to find impartial information that isn't full of either right wing, or left wing rhetoric.
To me I think it's such a huge decision to expect the general(ly misinformed) public to make, especially with the current terror / refugee crisis.
Obviously the EU isn't perfect, and needs a lot of work, but I really don't think "we" as a country are strong enough anymore (we're aren't the empire power of 60 years ago) to stand alone.
I can see it going the way of out, and that worries me.