0. doesn't use zlib, so to use .xp files, you have to use 7zip to extract their contents first (i.e. right click, extract here). If there's no compression anywhere, this will result in a slowdown if your .xp files are large.
1. only reads files. no saving. no transparency tests. no flattening. no creating new RexFiles. whereas REXSpeeder can do those things.
2. header-only: include the 3KB file and that's it, no need for configuration changes or libraries
3. no exceptions. instead, dumps errors in the std::cerr stream and then abort()s, so in case you're wondering where a bug is in your program, then wonder some more because this library sure won't tell you any useful information
4. if the .xp file is maliciously crafted, and you try to use the vector<> layers directly, then you are vulnerable to a buffer overflow. using getTile() is safe. I'm too lazy to fix this at the moment.
License: I place my changes in the public domain. This builds upon the existing REXSpeeder library, which is MIT-licensed. But the two libraries are only superficially similar now.