Difficult to build native .Net projects without using Mono, but that's more of a disappointment with the .Net build ecosystem and not a fault of Travis as such.
Also, pricing - I've only had need to use the free open-source tier, as all the projects I've worked with are open source, but the price of the first paid tier ($129 per month!) is, for me, quite prohibitive and really puts me off making the leap to pay for it.
If there was an even cheaper tier, for example one which was say $30 per month with 1 concurrent job and limited minutes per month, but allowed private repositories, then I would probably make that jump. I just feel there's a big gap in the entry-level pricing tiers. Review collected by and hosted on G2.com.