It should never sound like the English "r" (which is, by the way, a very unusual sound; Mandarin is the only other language that I know of that has a "retroflex r sound")..

When "d" is intervocalic (or at the end of a word) it can sound very much like the "th" of English as used in "them" (the similarity of sounds also accounts for the Brooklynese "des, dem, dose" pronunciations). In a number of regions the "d" of "-ado" / "ido" (common as terminations for past participles, is often not pronounced.

For the "standard" pronunciation of "d" in Spanish, consult this pronunciation guide (under "occlusivas")

You are DEficient! Haha just kidding. Depends on the country where it's spoken, I've heard (from songs and movies depending on the region) a normal sounding d to more of a kind of 'th' sounding soft d. Kinda like "días" sounds almost like "thee-ahs".

I think it depends on the word, who is speaking it, and what region or dialect/accent they speak.

Try listening to the word Madrid in English and then in Spanish.

The English pronounces the 2 Ds whereas Spanish sounds more like "Mathrith" For my ears anyway.

Samdie said:

In a number of regions the "d" of "-ado" / "ido" (common as terminations for past participles, is often not pronounced.

I've often heard Puerto Ricans say pesc'ao (pescado) and cuid'ao (cuidado). The also drop their "s" in many words, i.e., ¿Cómo e'tas?

This isn't exactly have to do with D, but here is an example of how different people pronounce letters/words. BEBE from spain sings the song Buscome, but if you listen she drops the S so it sounds like bucome:

http://www.youtube.com/watch?v=8Sb19Pjdt70

Seems many from spain do this with the S in words.

You're not alone, English speakers do find them quite challenging. I guess you also find it interesting how the hard g (as in gato) is pronounced, at least in Spain. It's like a mixture of g & k.

place the tip of tongue on upper teeth. To see what 'am talking about go to http://www.uiowa.edu/~acadtech/phonetics/#