At 99 you have a lot of options. You can either pray to get the highest ilvl items while using 3 fully upgraded crafted pieces (Kinda boring), or you can use ANYTHING in the game and it will scale up. You could use a level 10 item and it will scale up to 99. This creates a lot of fun possibilities, such as all the low items with bonus armor. Stacking those on a tank can be pretty fun. A lot of tier set bonuses will work too (But not all of course), leading to some interesting combinations. Same deal with old trinket/weapon procs. Remember that haste mace from Gnomeregan, the Manual Crowd Pummeler? Well in the 99 bracket, it scales from 193 to a little over 7000 haste.
Overall, unfortunately, a lot of this will be changed in Legion though. Twinking in general will either be hit or miss. While there are some big negatives such as stat templates in pvp, there are also some interesting positives such as xp off and on players no longer having their own brackets.
So who knows!
Edit: Ok not everything scales so well, and some procs/bonuses don't scale at all. Just felt like adding that lol. You gotta experiment a bit.