ALL REVIEWS

While the interface for this dictionary is the best of your dictionary options here, Merrian-Webster falls down when it comes to the all important content. Many words, notably those obscure words that nearly everyone has to look up, are not listed.
Also, the definitions for those words that are in the dictionary often exclude common meanings (try looking up "bias", according to the Merriam-Webster app, prejudice is not a meaning).
Still, this app excels in identifying multiple forms of a word (i.e. 'run', 'runs', and 'running'). Something that all the other dictionary providers seem to fall down on.

Has limited definitions and a limited vocabulary, which is frustrating - having entered what you know to be a legitimate word, only to find it does not have it listed. Also, the whole thing regularly will not load. Frustrating.

I agree totally with the Australian reviewer, Luke Stevens, though, to be fair, when I looked for Word 2013 apps I was told that there weren't any for my country (England)! 
Consequently, as Mr Stevens points out, the dictionary recognizes neither the Queen's English nor Australian English - try putting "colour" or the like into it, let alone any of the Australian words he lists. It an American dictionary, for undemanding American users, and parochial for all that.
One would have thought that a program with a name like Word would be first and foremost concerned with establishing a powerful dictionary. The Oxford, which serves the Kindle so well, would have been a boon here. However, non-American and American academic users are currently stuck. The bing dictionary is superior to this one, but (as the reviews suggest) it is still lacking.

1) The dictionary task pane does not provide a pronunciation guide; it does not provide “part of speech” information (e.g., noun, transitive verb, adjective, etc.); it does not provide synonyms, etc., etc. If you require information other than the few meager definitions this app provides, you must open/use a web browser window to obtain it (or just use an actual dictionary text). In other words, if you must open a web browser to obtain the dictionary information you’re after, there’s really no reason to install this Office app.
2) On my computer, Word 2013 hangs if I leave the dictionary task pane open for a few minutes. I literally have to kill Word via the task manager (CTRL+SHIFT+ESC) and re-launch Word to continue working. And of course, Microsoft provides absolutely no obvious means for removing this or any other Office app after it is installed.

UI needs some improvements.

good definitions, however synonyms are limited and does not define the different usages of the term ie. verb, adjective, noun. It only gives the meanings, not the type.

It doesn't work. I've installed it twice. It tells me my internet is down and gives me an error message. The Bing one works. This one is terrible. And why do I need to download and fool with an app for a feature that's been a part of Word for the past 25 years? It's just crazy. It's an opportunity for some developer to make a decent app, but it's a shame that Word shipped with no dictionary and poor support from MW. Boo.

I guess having it in-line is ok but most of the real results are on the web site so it doesn't help all that much

The dictionary is either always there on the right side of the screen or you need to load it up to look up a word. Therefore it is either obtrusive or about as convenient as using a web browser. In reality this is the same as the Research button from Office 2010 but with only one dictionary, no thesaurus and no web search. Oh, and you need to download it just to add that extra inconvenience. 
Also, the version I downloaded was supposed to be Australian English (yes, that is a thing) but it didn't recognise thunderbox or dunny and those are just some synonyms for toilet. There are far more obscure words out there that a decent dictionary would know or, in the case of the old Research button it would be able to search the web for them.

Only gives some of the definition and explanation. If you want all the info you still have to go to the website. Also, no thesaurus. That seems like an easy function that's just not there.

The app works great in word but is unavailable for use in Outlook 2013. Furthermore, Microsoft seems to have seen it appropriate to remove the Encarta dictionary from outlook as well, leaving you with no option to define words while composing/reading emails!? Please make this app compatible/available for Outlook 2013 ASAP.

It's great to finally have the web jump out from inside of Word, to be able to use online services for in-document lookup of definitions, without leaving Word is a great idea.

This is a cool dictionary app that makes looking up words much simpler.