Generally, a subpoena must be personally served to be enforceable or some personal acknowledgement that it has been received. Due to the sheer number of cases, it is too expensive and time consuming to personally serve every potential witness in every case. This why they mail subpoenas and hope people respond. Cases that are important or likely to be tried, subpoenas will be personally served.
If the subpoena was simply mailed to your address over 2 years ago, it is likely that nothing will happen to you now. If the subpoena had been personally served upon you (rather than simply being mailed), then that would tell me that whoever was doing the subpoena-ing cared a whole lot more about you showing up to court to testify.
Commonly, subpoenas may be mailed out to "friendly" witnesses who will appear in court without much prodding. If there are "hostile" witnesses that one wishes to compel a court appearance out of, then one would arrange for personal service. If someone who was personally served with a subpoena does not respond, the court can potentially order law enforcement to bring in such a person.
Hope this answers your question.