Took a while to read this thread!
I too like many others, it seems, am guilt of the 'nice shot' comments, but I try and add something more to it - such as 'maybe increading the constrast might help make it stand out more...' or something along those lines. I am no expert but I have learnt a few things when it comes to processing and feel I can often comment on that easier than I can on the photograph itself.
As for how I decide how to crit a shot (if at all) I tend to go on first impressions. I opena thread...if it is something that interests me or catches my eye I will look at it some more, and decide if I would have done anything different myself. I dont look at it technically to start with I purely go on if it makes me go 'oooo thats good' - so I guess simply I could just reply with that (and I probably have) but if there is something I would maybe have done differently then I would generally say so. Before that though I have a look and see who posted it, if it is someone with a prettly low post count (say under 100) then I will try and make the comments more jolly...so that is doesnt seem like I am 'ripping it' - if it is a name I know I will generally be more 'honest' and pick out any minor bits too.
But overall I only tend to comment on photos that capture my interest in the first place. I would love to be able to comment on every photo posted, but there just isnt the time...and I am probably one of the people on here the most (it is generally open in the background while I am working).
On the other hand, if a photo is in one of the critique sections then I will generally try and leave an honest critique of what I like and what I would do different....no matter what the shot is. I really like the anonymous crit section as I know my comments are normally based on the original poster.
(hands are starting to ache now!)
Interesting thread though

and it is interesting that some people dont feel they have enough expertise to comment, which is think is entirely fair....and I think getting 'nice shot' from someone reletively new to photography is great

and then again I do like getting similar comments from people who's photography I respect....so I guess it is just a matter of opinion on what is a good response to your photo. I always like some constructive feedback, but as I have said before I always find I have to be carefull with how I respond to it, so as not to come accross as arrogant in my reply - I have often looked a photo for a long time before I post and any flaws etc I have already spotted...and generally accepted - yet a reply stating that can sometimes be miss-read as arrogance, or so it would seem. And I think that happens to a few other people too.
Ok I have to stop typing as I have written so much now I cant be arsed to read what I have put so it proably makes no sense what so ever
Conclusion: I like any comments on my photos - wether it is just a 'nice photo' or a propper good crit or anything in between. I worked out a while ago that on here you dont get many comments for just 'ok' photos - you get helpfull comments on 'bad' shots and some 'wows' and 'nices' on 'great' shots. I still think this place is 100x more helpfull than places like Flickr where a comment generally is just that.
Viva La Talkphotography

(and stupidly long posts)