Actually there's a lot of bad art in the WOTC books...it's just covered up by color; often badly done digital color.
What 3rd had, to a great extent, was a consistent feel.
4th Edition, at least while I was keeping up with the line, was just an embarrassment. A lot of it was technically amateurish, and the rest was just aesthetically...meh.
It might have changed. I remember being struck by an illustration from one of the Essentials books while I was flipping through it at the store.
One can forgive a lot of the mediocre art in the early books (pre-1982 essentially), because you're talking largely about a company that was working on a shoestring. That they had any art was pretty amazing. The 1982 Mentzer Basic set was really the first go TSR had of trying to put out a product with a consistent look and feel, and it's still flat beautiful. And it stayed that way troughout the line. The D&D Companion has some of my favorite art ever published by a gaming company, along with Star Frontiers and Gamma World 2e.
The Advanced line had to wait for a makeover until 2nd Edition, and while I'm not a fan of most of the color art for those books, the overall look and feel, the layout and design of the books is great. And the 2e Monstrous Compendiums are still the top monster books of all time. And a lot of the ancillary materials from that time (the Forgotten Realms material for instance) had really great art.
3e is close, but I don't think WOTC has ever matched TSR at their best (mid 80's through early 90's) at putting out beautiful art.
From what I've seen of the art for Next, the art for the monsters will be great, and the art for people will be very boring.