ZIP is seekable -- you can get a list of files and/or read individual files out of the archive without decompressing the whole thing. TAR+(stream compressor) isn't; the only way to get a list of files or to decompress individual files is to read through the whole thing.

The 7zip file format is a nice middle ground in this regard. The file has an index, just like ZIP, and files are compressed either individually or in small groups ("solid compression"); either approach makes it practical to extract individual files.