In a few words, MP3 Diags is a GUI-based application that allows end-users to identify issues with their MP3 files, fix some of the issues and make other changes, like adding track information. It also lets you "look inside" an MP3 file.
Unlike some programs that are designed to deal with a single topic (like fixing VBR headers or adding cover art), MP3 Diags is a one stop solution that identifies more than 50 different issues in MP3 files and provides the means to fix many of them.

Features:

broken tags / headers / audio

duplicate tags / headers

incorrect placement of tags / headers (ID3V2, ID3V1, LAME, Xing, ...)

low quality audio

missing VBR header

missing track info / cover art

broken track info / cover art

missing normalization data

character encoding issues (for languages other than English)