I would argue that classic Arminianism is less heretical than modern Arminianism. Jacob Arminius understood pelagianism and did everything to attempt to differentiate his beliefs from the pelagians. His view of prevenient grace for example, though still wrong, was a way to explain that God must work in a persons life before that person can show any inclination to accepting Christ. It is the modern semi-pelagian Arminianism that I find heretical.
As far as dispensationalism goes, I understand that they hold to orthodox beliefs about the gospel, but my concern is that they serve a different God all together. To suggest that God would set aside the sacrifice of Christ, accomplished once for all the elect, and turn again to a sacrificial system shows a complete lack of understanding regarding the atonement. They also ignore the urgency for people to accept Christ, giving them 7 years after a "rapture" to turn to Christ. The Bible is clear that when Christ returns that is it, there will be no more time to turn to Him.
Sorry if I have hijacked this post. I still agree completely with the OP, the term "heresy" is thrown around way too often. Perhaps it would be better to point out specific heterodox views of Arminianism and Dispensationalism, rather than declaring all of those entire systems to be heretical.