Just like in any major battle or issue there has ever been, people have a tendency to pick sides and stand firm to their convictions.  I've come to find that when it comes to sides as in food people can become just as fanatical.
I recently conducted a little poll over the issue of coleslaw. The question was, do people prefer it on top of their BBQ meat in a sandwich or on the side.  The numbers were close but, "on the side" won out. When I asked my wife about this, she said on the side because that way she didn't have to eat it, I had not taken this into account when conducting my poll.  The surprising thing about this little topic was how much those who liked it on the top of their meat stood behind it as some sort of belief or principal of proper BBQ Sandwich construction and eating. (I too like my coleslaw on top when available but, that is beside the point.) Unlike the ones who wanted it on the side, these people were quick to give reason as to why it should be in the sandwich instead, some said it was for the contrast in texture, some said it was because of the blending of flavors, and one simply said "The way god meant for it to be". 
All of this got me to wondering how strong people feel about their sides when it comes to food. Just thinking of all the different sides one can have with BBQ alone could cause ones head to spin but, I'll just touch on the big ones and what I think they bring to the meal. Since we have already discussed coleslaw we shall skip past that one and move onto potato salad.
The issue of potato salad in itself can be rather controversial. The options of what to put in potato salad can lead to fist fights in some families over the issues of onions and celery to even more diverse ingredients.  Aside from that issue there is also the fact that commonly there are 3 different types of potato salad. You have the standard yellowy looking just-like-mom-made kind, the steak house sour cream with baby reds style (my personal favorite.), and the vinegary German potato salad. Of all these types I would have to say that when it comes to BBQ you have the yellow and steak house as the most common, not that the German isn't excellent in it's own right just don't see it at many cook outs.  I believe that the one thing that it can be agreed upon with these two salads is that their creaminess and sweetness is what usually makes them a great compliment to the spicy and vinegary taste of some BBQ. Though personally I think that the standard yellow can get too sweet at times causing it to over balance out the BBQ flavors.
Corn on the cob is used a lot at BBQ restaurants as a side often.  When it comes to cooking at home I think it is often underused.  The light, crisp, semi-sweet, flavors of corn on the cob blend well with BBQ because, it has almost a pallet cleansing property to it and doesn't override the taste of the BBQ.  Also with the edition of butter, the salty and creamy undertones balance well and can help to heighten the BBQ flavorings. In the end I think corn on the cob is under appreciated at home and underused on the grill.
Pasta salads, just like potato salad can be extremely diverse but, they are a staple of almost every cook out whether it be a tuna salad, Italian style, or any other kind you can think of.  Much like potato salad, the induction of an mayo or salad dressing type pasta salad always seems to produce some pleasing balance with BBQ however, just as with the potato salads, you can get really overloaded on the sweetness factor. When it comes to the vinegar based pasta salads you really have to know the meat you're serving. One could easily go into vinegar overload if not careful. When it comes to pasta salads in general I think the underlying factor of their popularity is either in the induction of vegetables to the meal or the pasta itself giving a texture difference to the BBQ. No matter what though, pasta needs to have a nice snap to it though, please no over cooked gluey pasta dishes. People come to eat, not make art projects.
Baked beans are very popular, from what I'm told. Personally, don't come across this too often. I think it's because most people that I know find baked beans to taste too much like BBQ already. However, I think this has mainly come from the fact that most people get theirs from cans and don't make them their selves.  When you try homemade baked beans you find that their flavors can be extremely diverse and I believe if done properly, you can make baked beans as a side for your BBQ and have two totally different flavors coming from each.
The last one is macaroni and cheese. Yep, it is a very popular side. I think the reasoning is pretty understandable given the points of popularity concerning the other sides mentioned above but, I'll still mention it.  Creamy, salty, cheesy goodness blended with nice firm pasta. Makes me hungry just thinking about it. It's almost an exact balance to BBQ, creamy vs. astringent, subtle vs. intense flavor, and salty vs. potentially sweet. It's easy to see why it's popular but, the only word of caution I have is, be careful of the cheeses you use. Some have very dominating flavors and could be a meal just in themselves.
In the end, when it comes to sides for BBQ, there are almost too many to name and, I know I have barely covered the tip of the ice berg. I'm sure I will be getting told which ones have been missed and why they are better than the others mentioned. Truth be told though, my personal favorite is Au Gratin Potatoes. The combination of them with BBQ is just heavenly to me. So when it comes to sides, people will pick their favorites and fight for them tooth and nail with anyone who disagree. Just like any war or political issue.  Lets just not forget the one middle ground we can all agree on though, the meat is what matters the most.