The reasons I thoroughly enjoyed Michael Mann's Public Enemies are the same points cited by critics who are passively dismissing it. Are critics so-so-ing Enemies because they expected it to visually resemble Bonnie & Clyde? Is it neither strictly conventional nor "arty" enough to please them?

Marion Cotillard in Public Enemies
It seems that more than anything, they would have preferred to direct the movie themselves, with their criticism limited to "if I had resources X and Y, I would have done something different if I were Michael Mann." Others like Daily Beast columnist & host of KCRW's The Business Kim Masters are generating a Fall of an Auteur story for themselves.
Most prominent among the nitpicky complaints is the assertion that the HDcam photography doesn't look right. I had the same knee-jerk reaction when I watched the first trailer, but I had zero complaints with it in full execution. It's not a glamour shot-filled, fine gloss picture as the pooh-pooh-ers "expected" of Mann. Did I miss the special request box? If so, I'd like to pre-order a three-plus hour Napoleon from Terence Malick with overnight delivery.
The cinematography is dark, verite, and unapologetically at elbow's length from the viewer at all times. It's uncomfortable in a very purposeful manner. Enemies is immersive, brutal, and raw, with the pop-bang-boom everyone is used to only hearing and feeling in war films.
From cinematographer Dante Spinotti as found by InContention's Kris Tapley:
"We wanted the look of Public Enemies to have a high level of realism, not an over period feel...Among the historical aspects are a lot of action, romance and drama, and Michael [Mann] and I talked about achieving an immediate feel."
The going template for gangster movies is to evoke sympathy for the robbers and murderers that serve as anti-heroes. The "bad guys" all turn out to be misunderstood Robin Hoods, canonized through a revisionist lens. Public Enemies isn't interested in making you fall in love with anyone despite their faults, the possible exception being Marion Cotillard's Billie Frechette. Not even "good guy" Melvin Purvis (Christian Bale) is particularly likable. I have a feeling that Enemies will do better than many may expect this weekend as an alternative, R-rated choice against Transformers's second week and Ice Age 3D.