I have been involved in this discussion before but since I have just returned from my second successful swap I feel I need to share this experience again!
I am on a website called homeexchange.com (If you saw the movie The Holiday it was based on this site). We swapped our vacation house in VT for an apartment in Florence. Two years ago we went to the English counrtyside, about 40 minutes outside of London. BOTH trips were incredible and there is NO WAY we could have afforded either trip for 4 people and 2 weeks without free lodging!
Couple things to consider....
Listen to your gut and what is impt to you. I am fairly anal and found the first house a little dirtier than I would have liked. Now I pay VERY close attention to the pictures and what is written. If possible, I try to get second homes as they are less "used" and tend to be less cluttered.
Do not assume your house/apartment is undesirable just because it is not in a "prime" destination. Play up the assets of your area, you'd be surprised.
Swaps are not always "apples to apples" as far as size/condition. My house is much larger than the apartment we swapped for but it is also not in the center of a city. Location trumps size many times.
Expect to send out ALOT of requests, sometimes unsuccessfully. Many people will not respond at all, many will not want to swap to your location. That's ok- you only need one!
You don't need to "own" a house to do this. Some of the most sought after spots are apartments or flats in major cities.
Can't wait to do it again :)!