Let's put it in black and white since giving examples didn't seem to help you two at all. Constructive feedback pin-points a problem and then offers suggestions for how it could be improved. Complaining finds everything that makes the person unhappy and claims it's crappy or that it sucks and does very little to point things in the right direction. EVERYTHING Luminis said was complaining. How valid the points are is irrelevant. Even if everything on there was 100% true, he still just said it was a problem and he wants someone else to come up with the ideas on how to fix it for him. That's complaining. The person who mentioned the XP and leaderboards in online racing isn't just saying "The online experience sucks." or "It's a mediocre experience". He was saying what exactly he thought could be done to improve it. That is called constructive criticism. See the difference?
He, just for the record: It was complaining, yeah, because that's what the OP asked for. In case you forgot the initial question raised by him:
I've complained about GT5 before but is it really worth moaning about because it gives you 1000 cars to play around with does it really matter whether their standard or premium
what is there else to really complain about
I answered that. Nothing more.
Want me to provide contructive criticism? Well, I'll keep it short.
"Out of those thousand cars (yeah, beautiful number, I know), 220 are up to date. The other 800 are just recycled, outdated stuff, restricted in quite a few ways. Oh, and they sound crappy. The A4 Touring Car my B-Spec driver is currently using to grind levels sounds like one of those little slot cars."
Solution: Outsource the work on the standard cars to upgrade them to second-rate premiums. Offer them in packs to download. Oh, and enable the user to pick himself which cars to be part of a single pack. Personally, I'd even be willing to pay a small fee for it.
"B-Spec mode and the incredible stupidity and inability of its driver."
Turn B-Spec into an alternative to A-Spec as it was in GT4. Probably impossible now, so reduce the 'training' factor. Patch the B-Spec driver's AI and behaviour to be more on par with the other AI drivers. He'd at least win reliably that way as soon as he has the superior car.
"The AI."
At least stop it from breaking way early. Maybe iprove the recoginition of the player's car a little, that's not as important, in my opinion, though.
"The experience system."
Increase the XP won in the events to reduce the grinding needed to level up.
"The lack of events in both A-Spec and B-Spec."
Hurr durr, add more of them? Otherwise, at least increase the amount of races for the events. That way, buying cars specifically for an event would be less of a waste.
"Lack of tuning options. Customising individual gear ratios and tyre pressure spring to mind."
Add more in-depth tuning. At least stop with the "Stage 1, 2, 3" stuff and do the engine tuning in the same way it was in GT4. As far as car setups go, at least do what Forza 3 did... hey, it's only Forza, how hard can it be?
"Cluttered menues."
Streamline them. Instead of stacking fancy pictures, go with what works. Also, reduce the amount of confirmation pop-ups for minor things, such as claiming a free car.
"Mediocre (at best) HUD."
1) Enable the player to resize and enable/disable parts of the HUD.
2) Place the information more to the edges of the screen. The worst example is the map during rally events.
"License tests are completely meaningless."
Make licenses the requirement to enter events again instead of levels.
"Events that require you to hunt for a specific used car, that appears every now and then and costs a fortune (and thereby stopping you from buying cars you like, unless you want to grind for even mroe credits)."
Either change the events to use premium cars, change the used car lot to provide these cars as you unlock the events or make them attainable as price cars.
"Damage (both visual and mechanical) completely lacking in GT Life."
Implement the mechanical damage from the online lobbies in GT Life. Just have the game auto-restore it for free like it does for visual damage. Make visual damage a little more severe, as in, not requiring a bazillion crashes to become noticible.
"Features that were mentioned on the back of the box or in the manual (at least the US version's) are missing from the game."
Dunno, like, add those feature?
So, there you go.