Similarities to x86
Although ARM chips are unlike x86 cores in implementation, they nevertheless provide a similar high-level programmer model. Both have relatively slow floating-point units (generating efficient code for x87 is a nightmare), but good vector units. Both are 32-bit, little-endian architectures. Both can handle unaligned loads and stores (although this isn't true for older ARM chips), but both suffer a significant performance penalty when they're required.
In general, C code from 32-bit x86 will work fine on ARM, without modification. The differences are subtle, and only matter if you're designing a compiler or writing an OS, or you care about performance.
Of course, if you're writing a mobile application, as you typically are if you're targeting ARM, then you probably do care about performance. It's worth taking a look at the exact features of the chip you're targeting. Even tweaking some compiler flags can make a big difference, and small modifications to your code can have an even bigger impact.