I used to be a diehard Nortel guy and now that I have been working with Shoretel for the last couple of years I am pretty impressed with it. It does have a few chinks in it's armor that those of us who are used to the big Nortel/Avaya PBX have an issue with, but overall it's a great system.
Having worked for a hospital for several years here are a few things that mau be an issue.
Scalability to the several thousand user capacity. Shoretel will scale, but it gets messy with server counts and such when you get into the several thousand, like more than 3000 arena.
Can't have the same number on multiple phones. You can do simultaneous ring, but it's very limited at this time. You can also do hunt groups and basic ACD stuff to get around this, but there are limitations on that too.
This is my biggest pet peeve, very very limited access to granular control over the dialing plan. However, if you are not a multi-site/campus environment then this is a non-issue.
Having said all that I still prefer Shoretel over the others for ease of install/admin, overall price and value. You get a lot of bang for your buck out of the box with Shoretel.