Well, to go back to the OP's point and talking about what we see/do in Enterprise - traditionally apps like SQL/Exchange would have drives for OS, DB, and Logs. Virtualized that becomes less clear as they're usually on the same SAN, but often still done just to handle queue depths per virtual disk and such. You put SQL *AND* Exchange together, PLUS put all the rest together, and you really need to have an SBS that has 8+ drives in RAID10 or RAID5 (even if only to get enough spindles/pack-rat space).Lots of really good advise in this thread. As a side note, I've never been able to get 2011 running acceptably in any environment what-so-ever. It slaughters the storage system... 2012 R2 is a massive step up. I agree with those recommending SSD storage here. Just would recommend getting some stuff with decent write endurance and performance consistency if you're going to do anything with SQL on them let alone Exchange.
Too many times these things are tossed on a mirrored (or single ) SATA disk that you wouldn't use for ONE of the roles, but people hope ALL the roles will run great on. It totally needs a ton more disk IO than most people give them. The nice thing is that if you compare even Intel Enterprise-y SSD's to 10-15K SAS dissk for cost, you can build a pretty decent disk subsystem for about the same price with all SSD. Give a good 4C/8T/64GB/All-SSD system to SBS, and it'll preform pretty good - what choice does it have at that point?