Hiya.
With regards to the whole "player expects X, DM says Y...now player is screwed/upset" thing.
If a player has a character that relies primarily on some particular "schtick", the moment his butt hits the seat he should ask the DM how certain rules are applied. For the 'thunder cleric' example mentioned above, the *player* needs to ask the DM how he interprets X, Y and Z. For the thief trying to do the shadow-attack-backtoshadow thing...
definitely ask the DM. If you are playing with a new DM, s/he doesn't know what your characters method operandi is...so it's your job to inform them so you all know what to expect. If it turns out to mess with your perfectly chosen skills/stats/feats/whatever, then you have time to adjust your play style...or at least you know what to expect.
Besides, if someone has built a character around some particular "build", "tactical method", or "rules exploitation"...then finds himself in a game where that's just not going to work because of the DM...well, that's not the DM or the rules being "bad". Maybe the player should have concentrated on making an interesting and in-depth
character, and not just a collection of specific stats and abilities. If you want to do the whole "perfect build" thing...5e isn't for you (generic 'you'), go look at 3.x/PF/4e (it seems to be built perfectly for that).
As an aside, I think the way the rules in 5e are written is one of the
best things they did, design wise. What this will do is teach DM's to actually
DM and not just "run the monsters". I foresee the caliber of DM's rising significantly over the next three to five years because of this. I can see it also improving player skill, as right now player "skill" seems to be all about how well one can stack numbers; getting away from all that 3e+ stuff and back to a more 'loose and organic' style of fantasy role-playing can only improve player capability, DM capability, and campaign fun. IMHO, of course.
^_^
Paul L. Ming