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First off I'd like to say I have a 50 Cleric, have cleared all T1 and T2 expert dungeons and 3/5 currently in Greenscale's Blight raid instance, so I have actually played the game.
That said, I just wanted to give a warning to those playing the game, especially possible healers. Oh and this is strictly PvE.
UI: While you can modify it to your pleasure, it still lacks a lot for a healer. You can't see only your buffs or heals on your target, so you get a line of 5-6 buffs on your tank. Playing a Warden(HoT based healer) is an utter pain since you have to find your HoT stacks in the middle of all those buffs. Playing Sentinel or Purifier instead, you don't have to pay much more attention to your tank's buffs, but your healing mostly consists of spamming your 3s induction heal, because other heals are largely inefficient.
Also, you will see debuffs on your tank's raidframe, but you won't be able to know what debuffs they are, so you have no idea if you have to cleanse it or if it's not worth wasting a 1.5s global cooldown instead of simply healing through it. Not to mention that other class debuffs are listed there too, so you may not be able to cleanse them off anyway.
Other than that, it's ok since you're able to modify at your will, although group frames are much less efficient than raidframes, even in 5man content.
Gameplay: Like I said before, Sentinel and Purifier rely on their 3s heal spam, making for a rather boring healing gameplay. Sure you have some long CDs, a battle rez as sentinel, 1-2 45s CDs for a bit of burst, but you're deluding yourself if you think you're doing much else. Sure you can hybrid spec, but most of your heals are just another variant of the same one, and 36+ points into a spec give quite a boost to your shields as Purifier and to your main 3s heal as Sentinel.
Warden on the other hand needs you to be a bit more active, keeping your HoT stacks up on the tank, but between the UI and just not doing much else; as well as lacking burst capacity should you get CC'ed and lose your HoT stacks, it's not exactly thrilling either.
One last thing to mention is when your group is taking major AoE damage, the cleric only has so many tools to deal with it, which causes you to rely on your support class of choice to really help you as you can only do so much, with Sentinel having a 2s AoE heal as your spam AoE heal, with 1 1min CD AoE and 1 major 2min CD AoE; Purifier only AoE heals with your overheals on tank, which is not always possible; and Warden has 1 spam AoE HoT which doesn't help much in spike damage, 1 2s cast medium AoE heal, and 1 major channeled1min CD AoE and 1 2min CD AoE HoT heal. Notice this is with the full 51 pts in your spec, so hybrid has less tools for AoE healing. They may seem like a lot, but you also have to know that AoE healing is a major mana burn, so don't think you can actually spam them. (When I called an AoE heal a spam heal, I meant as it has no CD besides global cooldown).
Then there's Cleric DPS. As melee, you mostly pick between Shaman and Druid as your main spec. Sadly, you can macro your abilities so that you have 1 button to spam for single target dmg and another to spam for AoE dmg, with a few random CDs. Not much else to say here, once you know what to put in your rotation macro it's quite silly tbh.
Tanking is your generic tanking with generic tank skills. A single target taunt, single target spam, AoE taunt, AoE spam, no charge, no pull mob to self, 1 button to keep up your +20% block chance, and an AoE silence with 10s CD iirc. You basically heal yourself through damage, but again, it's lackluster compared to Warrior and especially Rogue tanks.
Overall, Cleric gameplay has to be the most bland I've seen in MMOs, and was one of the major reasons why I unsubbed.
Endgame: Eh. A lot of tactics are the same as the dungeons you did as leveling up, although a few mechanic or two are improved. The most annoying thing for a healer(imho) in Experts though, is unless you're overgeared, a lot of bosses require a Support class like Bard or Chloromancer(although the latter really mainheal quite well, with no mana problems, and their heals are AoE since they're based on their damage, as well as 2 instant heals, so no long cast heals which get you interrupted).
The tactics are usually your basic "move out of the dangerous pools of doom" or "cleanse spam" or "LoS this or that". Some have decent mechanics but nothing majorly complex - my favorite being Calyx the Ancient in AP with his lasers requiring movement the whole time as well as area awareness, or the dragon which I forgot name/area.
Then you have Rifts, Expert and Raid. While they were fun when leveling up, these versions mostly are tank/spank, and since you can just zerg them with the game not restricting people that can participate, they're just a grind for essences. The world invasions get a bit boring after a while(imho), but then again you'll end up with plenty planarite and blue shards by time you're 50 anyway. The purple shards are very rare, requiring major world invasions, but once you do manage to enter one, they're easy to get. Another thing to mention is Contribution is poorly balanced, your DPS does not factor your reward as much as spamming skills does. Most the time, healers will get highest contribution quite easily, then tanks, then random DPS, AoE or single doesn't really matter.
There's also dailies you can do, but their rewards are usually not worth the effort, since you get better items from dungeons than notoreity vendors, and crafting recipes are also not that great either. They take quite a while to get too, I was more than raid ready by time I had any considerable rep in the higher ranked factions.
Finally, after rifts and dungeons, you have raids. And sadly, this was another of Rift's major disappointments. For starters, they had to be buffed on release since people could clear them with barely any dungeon gear. Now they're a bit harder, but you don't have to be decked out in T2 dungeon gear to do them, and I've done raids with very undergeared raiders with little issue.
The raid tactics are rather fun to figure out, but their progression is always the same. First 25% is 1 phase, with 1 mechanic. Second 25% is another phase, with another mechanic plus previous. Third 25% is another phase, with two previous mechanics plus one new one. Last 25% is the "clusterfuck" phase, with all the mechanics put together as one. I mean, that's fine if 1-2 bosses are like that, but so far it seems all raid bosses use that method. I actually preferred dungeons to raids, even in difficulty factor(which wasn't that high anyway).
Overall, endgame was decent, but rifts and raids were very disappointing imho. For a game named "Rift", I expected a lot more from those. Dungeons are oky though, but that can only keep you along for so long. Oh and dailies feel like a useless grind.
There may be other factors, like graphics and crafting, but first off graphics don't make a game imho and they're usually personal preference, so people can decide that for themselves; and crafting is not worth writing about. It's only useful for when you just hit 50, but the good recipes from vendors you buy with plaques from bags you get from crafting dailies, and they're random so you may not get any for a long time. Also some crafting is way harder to farm for than others, like some Artificer salvaged components not even existing in game yet.
tl;dr I think that's pretty much it, overall I'd give Rift a 2/5; if you really need a MMO, go ahead, it's quite casual friendly(although be careful on which server you pick, some are a bit underpopulated), and can be decent filler while you await something else or are burned out by your current MMO, but I don't see it keeping people's attention for much longer, the gameplay just lacks a focus and is not that entertaining(Cleric wise for sure at least, although I have played other classes, just not as extensively), and the fun you have in dungeons will slowly wane and raids will not entertain you besides the gear progression junkies.
Forgive any typos and feel free to ask any questions if I missed anything.