I've voted to leave, here's a summary (more a horrifically long wall of text) of why that I wrote a couple of days ago, I'm on mobile so forgive any errors:

Immigration

The Single Market gives free movement of people between all member states, allowing anyone to move easily between any country in Europe for work. Free movement has caused workers from EU countries with few job opportunities and low minimum wages to move to richer countries (like the UK). In fact, many Eastern European countries face depopulation due to the number of people moving abroad to work, while countries such as France, Germany, and the UK face overpopulation as hundreds of thousands move there a year. Net migration (the number of people who arrived minus the number who left) into the UK is 330,000 a year and increasing, most of this is from the European union. Until the last couple of decades net migration has been around 50,000, until former communist countries began joining the EU (although net migration from outside the EU has also increased).This puts strain on public services and the housing market, and may lower wages and decrease job opportunities. Countries outside the EU have systems to control the number of people who arrive, selecting them based on skills the country needs, the UK cannot do this in the EU as free movement is not negotiable and only those who pose a serious threat to the country can be prevented from entering. Migration can only be controlled from outside of the single market, leaving would allow the UK to have a point based immigration system, still accepting workers needed for the NHS and with other skills we require, while reducing the number of people arriving.

Trade

As a member of the EU, we trade as part of it. Many argue that this gives us an advantage as we negotiate trade deals as one huge group of countries collectively worth more than any other market in the world. However, this also has disadvantages. The EU has to attempt to cover the different interests of not 1 but 28 countries, decreasing each countries say in negotiations. It also means that we cannot negotiate trade deals independently with countries we would like to, such as the commonwealth nations. The UK is the worlds 5th largest economy and countries far smaller than ours negotiate trade deals perfectly well outside of the eu. There is also concern about increases in tariffs with our trade with the EU if we vote to leave, however this is unlikely as the UK sells less to the EU than the EU sells to the UK and so raising tariffs would not benefit them at all

EU Budget

Each member of the EU pays into it a different amount, and each member has a different amount spent on it. The UK budget is around £18 billion a year, however a rebate of £5 billion is given back to us. The EU also spends £4.5 billion on programmes and projects in the UK. This means that the UK pays in £8.5 billion a year which we do not get back in any way, and the £4.5 billion spent on us is decided entirely by the EU. The rest of our contribution of the budget goes to other members of the EU, many of which have more spent on them than they contribute. Organisations that receive EU funding could in fact have more spent on them if the UK leaves as we will no longer have to contribute more to the budget than is spent on us. More could be spent on public services if the UK leaves if the money is spent in the UK instead of the EU.

Laws and Sovereignty

The European Union can create and enforce laws on its member states. It does this by having a European Commission of 1 appointed representative from each country create legislation, and a European Parliament that votes on it. The parliament is supposed to represent the views and interests of the 508 million EU citizens, however each MEP represents almost a million people, and less people vote in the European Elections every time one is held. Political views also vary hugely between European countries, and as turnout in the European Elections is so low, more extreme parties do well in them. Due to this the European Parliament contains both neo-fascist and communist parties. The whole of Wales has just 4 MEPs representing the entire country, and few if anyone even knows who their MEP is. This makes it difficult to hold MEPs to account and remove them from office if we disagree with how they have voted. The unelected European Commission can also be considered undemocratic as although nobody can vote for them or remove them, they are the only institution that can propose new laws. The number of our laws that are made by the EU each year is disputed however generally considered to be over 50%. The majority of our laws are now made by the EU, and the UK has little influence over them with just a small percentage of MEPs representing the UK. Britain is also the most often out voted country in the European Council of the 28 heads of state from each nation. Free movement of people in the EU takes away our ability to control immigration, making one of the main issues in UK politics out of the government's control.

Reform will not happen

Reform of the EU has already been attempted, in February the Prime Minister held a renegotiation of Britain's membership of the EU, and, despite the UK future in the EU at stake, was not given anything substantial at all. Cameron has promised to lower net migration "to the tens of thousands" despite its constant increasing over the last 6 years, however renegotiation of free movement of people was ruled out before negotiations had even started. What we instead got was the ability to not let migrants claim benefits for the first 4 years, which is completely unnecessary as migrants actually pay into the system more than they claim. The renegotiation failed to address any of the UKs concerns with the EU, and President of the European Commission Jean Claude Junker has said that there will be no further reform if the UK votes to remain.

I imagine the majority on reddit will have voted to remain and that most of the answers here will show that, just thought both sides of the argument should be seen.