My wife was granted a conditional permanent resident status (green card) in the US and we started the process for removing the conditional status while we were living in the States after being married for more than two years (as per the requirement). Before receiving a response to our application, we made the decision to return to her home country for work reasons. We recently received mail from USCIS requesting further documentation to prove the "legitimacy" of our marriage. The deadline to submit the additional documents recently passed, and I had already decided to give up pursuing the application process anyway, since we will not be returning to live in the US for at least five years. My concern (and question) is this: Can my wife still enter the US to visit my family, or will the lapsed/failed green card status create an issue that prevents her from even entering the country?
No, she'll have to apply for a visa of course but there's no specific limitation on ex-permanent residents from visiting.
However, you should have cancelled the green card instead of ignoring the USCIS, as she's still liable for taxes in the US and her status is in limbo. The proper way, IMHO, would be to go to the US consulate and officially surrender the permanent residency using form I-407. Do talk to an immigration attorney to make sure.
To provide an update and back up the answer provided by littleadv:
My wife completed the I-407 form, went to an interview (scheduled) with the US embassy here in Korea, and relinquished her permanent residence status. The official gave her back a signed copy of the form showing that she had given up her status. A short time later, we applied for an ESTA so she could visit the US and it was approved. When it came time to actually visit the US, the customs and immigration officer reviewed her papers and admitted her without trouble.
I realize this is anecdotal, but it does confirm that, under normal circumstances (i.e. no criminal history, overstays of past visas, or other irregularities), former PR holders can re-enter the US as tourists (and even get an ESTA).