If you work in the environmental sector as I do, then opting out of the EU Directives on air and water qualtiy for example would be a disaster - the UK has done quite well with these issues in the last few years - incredibly important for heatlth and well being, and knock on effects for the NHS and cost of living (we have a cheap and readily available water supply) etc...
Leaving would also erode the rights for equal pay and minimum wage etc..
Staying in offers the opportunity to push fo reforms - which are needed, and not just on immigartion (which is not the key issue - the economy is), but things like agricultural policy - again we want not maintain affrodable food I'm sure.
We would end up competing with Europe in a global market, rather than working with them.