I have a "fly tying chest" and a rolltop oak desk. The chest was built specifically with flytying in mind - several wide, deep but shallow drawers lined with aromatic cedar. The rolltop wasn't built with fly tying in mind, but has been taken over. When I built the chest, I also had a "lap desk" - It had a raised section to mount my vise, a magnectic strip to hold hooks, dowels to hold spools, and holes drilled to hold tools, bobbins, scissors, etc. I still use it with the roll top - and I take it with me when I travel (if I'm traveling by auto).

So, my one bit of advice, if you're building your desk specifically for fly tying - roll tops are great for keeping cats (and kids) out when you're not tying, and consider aromatic cedar for some of your material drawers.