Good kata!
Still, I'd prefer to see example with -x and big numbers in initial unit tests.
For example:

Assert.AreEqual("-x+1", KataSolution.Expand("(-x+1)^1"));
Assert.AreEqual("y^15-75y^14+2625y^13-56875y^12+853125y^11-9384375y^10+78203125y^9-502734375y^8+2513671875y^7-9775390625y^6+29326171875y^5-66650390625y^4+111083984375y^3-128173828125y^2+91552734375y-30517578125", KataSolution.Expand("(y-5)^15"));                             

Sorting the list just to find the element is doing O(Log(n)) complexity task where O(n) task is totally enough.