In my opinion this can be broken down into three elements:
Riding / Horses                   Accommodation                                                People

Riding: You need to choose a holiday with the type and standard of riding to suit you.

Horses: Most places have a variety of sizes and types but welfare is most important.

Accommodation: This can range from the palatial to the
(a) scruffy because, let's face it, horses are more important than people and housework!
(b) Scruffy because standards are generally low

Of course you may not be able to tell from brochures and different clients have varying standards.

People: These, in the end, are the most important, assuming horse welfare is good. They can make or break a holiday. In my experience, if they are not very friendly and helpful on the telephone they are even worse in person. Of course if you phone them every day, late at night with a new query they may get a little impatient!