After playing on thinking about 5e...I'm on the fence. I will say I am a much bigger fan of 4e than the prior additions. So I am biased against a nostalgic approach I'll admit. That being said, I don't think I've seen enough to say I wouldn't play it.
My group really liked the advantage/disadvantage. The GM said he had a few concerns about the potentional un-hitability when you stack defense abilities with it. But still really liked it on the whole.
As for wizard, the wizard player and myself was never fans of 'pick your spells for the day and hope you don't get any nasty surprise encounters.' We're considering house ruling it to a mana pool system, but I hear that was not uncommon in 3.5? Our wizard player prefers a more dps heavy spell caster and did not enjoy a more control focused caster. That's a character creation complaint however and not a mechanics issue.
The Fighter, I don't know about other people's experiences but it tore the hell out of mobs. Spine out the back ala Predator, I will paint my face with your blood.
Rogue we didn't play right. There was a hard time remembering the new rules for it so it ended up feeling under powered. Also the way the dungeon was laied out we did not find a lot of good situations for using him as an ambusher/skirmisher. That's a more situational complaint though. If we generated our own landscape/encounter that wouldn't be an issue.
Clerics...eh? I was never a fan of the class in 3.5 and 4e so I'm not sure how I feel about them. They seemed to play fine from what people said.
The Background & Theme feels like it has a lot of potential. A Fighter can be a beserker, tank, pitfighter, brawler, instead of making 4 seperate classes that are essentially a re-dressing of the fighter.
Healing is a big concern, at least for my group. We never have 5 players and no one likes to play cleric. One of the things our group really loved about 4e is class flexibility, you could have two players that had some healing abilities and you could move right along without too much trouble. Since we haven't seen character creation options or stuff beyond lvl 3, I'll have to wait and see how it pans out.
There's a few other minor things, like we're going to house rule potions to be 4+1d4 hp and get rid of electrum?
On the whole, on the fence. A lot of my groups concerns are character creation related and whats beyond lvl 3. Looking forward to the next phase of the playtest.