Basically, for shred guitars, one is looking for the following:
1. Flat fretboards (16 inch radius is the most ideal)
2. 24 frets o di kaya 22
3. Humbucker on the bridge pickup position
4. Floyd Rose or any locking trem system
But then again, Malmsteen uses a strat.
But the above enumerated are the rough guidelines in buying a shred guitar.
Washburn, Jackson, B.C. Rich, Ibanez, Charvel - those are the most common brands in shred guitars. They were first called SUPERSTRATS.