Jeffrey put it well; I'd add that many are repulsed by the antics of rabid Wagnerians and all the other stuff surrounding Wagner and his "tradition" (Winifred, Hitler, whatnot) but all this has very little to do with the music. People who are put off by Wagnerianism tend to underrate the composer, I guess.
But I think that by many musicians, dramatists, composers and a few other artists (like Thomas Mann who was totally nuts about Wagner) Wagner is often highly rated and properly revered for really stunning achievements. Although even among such elite connoisseurs the hype might have been a little over the top ca. 1875-1915.