a) Your side does not validate. Font tags are so 20ths century, I don't believe it. At least, don't declare it as XHTML when it isn't.
b) Size based tag clouds are all right, but the 'most blue' doesn't really work as a good color for me.
c) Alt tags. Your images are all meaningless to a screen reader
d) those arrows up top seem to indicate links - but they aren't. Why are they blue then?
e) Same for the blue numbers ( 'n' vacation results from 'around the world'...)
f) http://www.rentalio.com/index.php?view=region®ion=Europe+-+Western cries for the use of mod_rewrite and pretty urls.
g) Google Ads within search results. That's your business model? You're not getting commision from the rentals? Anyhow, they don't belonge there looking like regular listings - especially with the images beside them. They at least need to be clearly marked.
h) More filters. When I'm looking for a location, I'm also looking for a certain size, a certain price range, etc.
i) Why do you keep repeating the country after each city ( at least for Germany ) ? I mean, it already says
"Showing 6,610 Germany Vacation Rentals from 178 Sites" ( note the horrible gramar in that sentence), why keep repeating it over and over?