MythTV creates large files, many in excess of 4GB. You must use a 64 or 128 bit filesystem. These will allow you to create large files. Filesystems known to have problems with large files are FAT (all versions), and ReiserFS (versions 3 and 4). The ext3 filesystem can be made to work but requires great care in how you format and mount the volume.
Because MythTV creates very large files, a filesystem that does well at deleting large files is important. Numerous benchmarks show that XFS and JFS do very well at this task. You are strongly encouraged to consider one of these for your MythTV filesystem. JFS is the absolute best at deletion, so you may want to try it if XFS gives you problems. MythTV .20 and above also incorporates a "slow delete" feature, which progressively shrinks the file rather than attempting to delete it all at once, so if you're more comfortable with a filesystem such as ext3 (whose delete performance for large files isn't that good) you may use it rather than one of the known-good high-performance file systems. There are other ramifications to using XFS and JFS - neither offer the opportunity to shrink a filesystem; they may only be expanded.
NOTE: You must not use ReiserFS v3 for your recordings. You will get corrupted recordings if you do.
Because of the size of the MythTV files, it may be useful to plan for future expansion right from the beginning. If your case and power supply have the capacity for additional hard drives, read through the LVM and Advanced Partition Formatting sections for some pointers.