I personally am against most collection features in games. Yes, they can give players something to do, but I also think they're usually added as an easy go-to feature with little thought involved. What does collecting plushies or collecting cards or stamps have to do with the game? Nothing really, but now the site can state that "collections" are part of what it offers.
I've also witnessed that they can be a way to take advantage of players. Sites with these types of unnecessary collections usually sell collection items in their cash shops. Someone that's spent many hours saving up virtual currency to complete 98% of their collection will likely spend the $15 on cash shop items to complete it. And then when another gets released in the cash shop, those collectors will open up their wallets again and again. Because no one wants to see 99% complete when they're worked so hard to collect all the other items. Yes, sites need to make money in some way, but I think this is just a form of manipulation. Because these items don't offer anything to the player's experience aside from giving them a goal of some sort (and goals should already be established in other ways if the site has any point to it at all). So it's this idea of giving "value" to something meaningless and making players believe that they have some value. And I just think games should have features worth paying for or worth putting effort into.
I am not against all types of collection features, don't get me wrong. But I like them to be things that are more than collecting site items that are just made for the sake of collecting. The kinds of collections I like and have liked in the past:
-Pet interaction collections (such as foods eaten or toys played with). However, this has become more common in sites and is almost getting dull. So I'd like to see a site that has it so these collections have a purpose. Maybe pets that have eaten more X foods than Y foods means they gain an agility bonus in battle. The opposite means they'll have more defense. If sites are going to bother to record pet interaction activity, they could make it worthwhile.
-Avatar collections. Who doesn't like these kinds of collections? Generally, avatars are how you show some kind of individuality and personality on the forum. So you're given something to "collect" while also have a goal you need to meet and getting something you can use indefinitely later. It's a perfect combination. I would, however, like to see sites moving away from the 60px by 60px Neopets-style, pixel avatars in favor of doing something more unique.
-Trophy and achievement collections. These are the kind of collections that make you earn them by completing a task instead of simply buying them. I like these, assuming the task involved takes skill and isn't just a reward for refreshing a page 5,000 times or something (I'm talking about tedious "do this over and over" achievements and randomized ones).