Room temperature. Well, that's what you always hear. The problem is that, at the very least, it is a bit inaccurate, and at the worst (as demonstrated by a whole lot of restaurants around where I live) you wouldn't want to drink it at 80 degrees Fahrenheit ("it's the room temperature, isn't it?")
As cool wine warms, vapors rise off the wine. Since your sense of smell is a very big part of what things taste like, getting those vapors into your nose is important. Try drinking a bottle of wine that has been heavily refrigerated. In some ways, it will taste a lot like water, or at least tasteless alcohol. On the other hand, if you serve a little below room temperature, you'll get the benefit of the vaporizing effect. So one rule of thumb is to serve the wine 1 or 2 degrees below room temperature.
But, there is a limit to the warmth. To some extent, you can use the following hints for:
If the wine is too cold, can you warm it in the microwave? I wouldn't think so, but one correspondent tells me that he saw (they call this hearsay, don't they) a notable wine expert do it with an old and expensive bottle, so . . . . Personally, I find that holding the glass with my hands usually gets it warmed up pretty quickly.
Call it scandalous, but I am quick to ask a restaurant to chill a red wine (gasp!) which comes to me way above a proper drinking temperature.