On the topic of whether "until the start of your next turn" is necessary...
For reasons like Total Defence and Second Wind and the like I think both are necessary, but I really don't find having a column each for those on a tracking sheet a big deal (I'm thinking mainly of DMing and tracking statuses for all monsters - besides that, tracking my statuses for one PC is frankly trivial).
I don't buy that total defense or second wind would be problematic if the defense bonus lasted until the the end of the next turn rather than the start. I don't think that'd be problematic
now in 4e as-is, and certainly not if the rest of the rules were designed to accommodate the change. The reduction in complexity is more that worth it - and tracking statusus for just one PC is not necessarily trivial, particularly for actions rarely attempted (such as total defense). I don't want to have to look up whether its until the end or start of your next turn. In any case, complexity for no good reason should still be avoided, even if it's not too serious. Is it game-breaking by itself? No. Is it good? No.
On the topic of durations until the end of your current turn:
Generally I think they ought to be "Start of your (= "affected creature's") next turn". You normally want those buffs when enemies attack you, and those you don't are usually useless if you aren't attacking them...
Not necessarily: I'm thinking of things like flying, speed bonuses, acrobatic rogue tricks, gaining some ephemeral advantage for yourself, etc. Consider also that I'd like to avoid a condition tracking moment at the start of turns (i.e. all effects resolve after turns, not before), this seems like an easy win. Defense boosts can last until the end of the
next turn. The extra duration during your own turn isn't very powerful, and certainly not if it's considered in balancing right off the bat. Given that the playtest doesn't seem to have anything like opportunity attacks, it'd be even less important.
As an aside, another element I think 4e got right was having all buffs be to final attributes, not to characteristics or the like. Boosting something that rolls on bonuses into all sorts of other things (like +2 Strength, for example, as opposed to +1 to hit - or even +1 to hit and +1 damage) just causes endless hassle and confusion.
Yeah. Although, I don't think there's any need to completely avoid these; they
do ensure a certain amount of consistency. For instance, some 4e powers say something like "... and shift your speed ..." whereas others say "... and shift 6 squares ...". I prefer the former precisely because it interacts nicely with things like slow and heavy armor; so here the "base stat" would be speed and modifications to it should ripple down.
Ability score bonuses are problematic anyhow. I see the flavor, and it
is sometimes appropriate - but rather than model bull's strength by attack and damage bonuses (but then missing things like encumbrance), I'd prefer making it very rare (i.e. no standard PC buff), make it last long enough to mitigate the tracking hassle, and include a list of likely consequences in the power description. In any case: as far as I'm concerned this should be a corner case. I don't want ephemeral ability score changes.
On the topic of save-ends being nice in theory but sometimes a little messy in 4e practice:
I know what you mean, but I think that was poor implementation, too. Just say that the effects don't stack but the saves do. In other words, if a character gets hit with three "ongoing 5 poison damage" effects, they only take 5 poison damage per turn (i.e. the effects don't stack), but they must save against all three (i.e. make three successful saves in total) before they stop taking the damage.
I think WotC just bottled out into thinking "this is too hard", here, when I don't think it's hard at all - it's entirely intuitive, to me and most of the folk I play with, and it makes a lot more sense in terms of monster relative power and so on.
Totally agree. I played with that house-rule for a long time; but some monsters hand out save-ends like politicians do handshakes, which isn't ideal. So, to really improve this you'd want to integrate it into the system from day 1. Say, isn't 5e around the corner

?