Also, make sure to detail what your data protection requirements are in terms of recovery point and recovery time objectives. If you're seeking a backup or disaster recovery solution at the same time, these will help ensure that the proposals match your needs. If you aren't, you'll need to go into detail about what your current protection scheme is to ensure that the proposals are compatible with it.
As you build your requirements, stay as solution-agnostic as you can. Stick with IOPS and usable capacity instead of particular sizes and types of disks. Also, avoid using vendor-specific terminology. So, if you own an HP EVA and are seeking an upgrade or replacement for it, don't say you need the capacity and licensing to support X number of Snap Clones per volume. Say that you need X number of point-in-time volume copies.
This may seem like an incredibly minor difference, but it has an important effect. Putting required functionality in terms of bare requirements rather than a vendor-specific technology will weed out respondents who don't understand the technology they're selling. Many potential RFP respondents are simply trying to throw together a list of SKUs at a price-point that you'll buy. If you want high-quality responses, you need to get past your potential partners' reflex to throw some stock solution your way and force them to determine how to ensure their solutions will actually work for you.
Be specific with support requirements
Be very, very specific about the support you want for any equipment you're considering buying. Support and licensing costs are an enormous part of many storage solutions. Because of this, you need to know what it will cost to support your solution throughout its entire lifetime as you evaluate proposals. Even if your company policy is to purchase support on a yearly basis only (which is usually very expensive over the long run), be sure to require that vendors specify what out-year support will cost. Some vendors fix the per-year cost while others will get more and more expensive as the hardware ages -- a very important differentiator.
Sort out pricing
As you review the proposals responding to your RFP, very likely you'll observe wide swings in hardware pricing from one respondent to another -- even when multiple respondents propose the same vendor's solution.