If your comment doesn’t show up straight away it will be for one of three reasons:
i) All first time commenters’ comments automatically go into the moderation queue, to guard against spambots etc. Once your first comment has gone through, the next should appear straightaway, unless:
ii) Comments with lots of links in are automatically put into the moderation queue as another anti-spam measure — an admin will approve them as soon as they are able, but bear in mind we’re not always wedded to the computer!
iii) Anything that is offensive, or considered to be spam, will be removed. Please note that there is a difference between ‘constructive criticism’ / general disagreements and being offensive!