Mechanically? Very little. There are a few minor details from 1E/2E that I miss, but not much. In 99% of cases, the new system is superior.
But there was a feel and attitude in the old editions that I occasionally miss in 3E. The best example of it, as someone else mentioned, was the art. As far as I'm concerned, no suit of spike-covered semi-punk armor is ever going to be nearly as evocative as the classic knight in plate. But leaving aside the details of the pictures, I miss the old style. I don't like the trend toward comic-book-style art. When it comes to fantasy RPG artwork, I prefer my colors painted, and I prefer most of the art be black and white.
Yes, I said I prefer B&W most of the time. When done well, it can actually be more evocative.
I miss art that told a story or presented a scene, rather than character portraits. With the efreet on the coer of the old DMG, with the City of Brass in the background--or with the picture of Emirikol the Chaotic riding through some hapless town--you knew there was a story there somewhere.