A fairly striking typological commonality between the “micro-Altaic” language groups: Turkic, Mongolic and Tungusic (Tk, Mg, Tg) is the lack of a labial glide such as /w/.
This is clearly out of line among both the world’s languages in general, and Eurasia in particular. /w/ is one of the most common phonemes in the world’s languages, that can usually be found even in languages with seriously impoverished consonant inventories such as Hawai’ian (at ZBB we  once compiled stats on this thing), and there is no shortage of *w in any of the other major language families hanging out nearby: IE, Uralic, Semitic, Dravidan, Sino-Tibetan, Austronesian, Eskaleut, you name it. Even in languages that lack /w/ precisely (Finnic, Slavic, most non-English Germanic…), it has usually not gotten too far off-field and has merely become a more frontal labial continuant such as /β/, /v/, /ʋ/. Yet none of these can be found in Turkic / Mongolic / Tungusic either. This clearly means that any long-range relationship hypotheses like Nostratic, Eurasiatic, Ural-Altaic will need to explain whatever happened to *w in Altaic.
There are two main hypotheses going around that I know of: *w > ∅ versus *w > *b. The former is the stance of some old-school Ural-Altaicists like Räsänen, among Nostraticists apparently Bomhard  and I gather also Illič-Svityč. The latter is the stance of, at minimum, Dolgopolsky. (He proposes also *w > ∅ before labial vowels in Turkic. )
I think the actual answer is neither of these, and the demise of *w is only post-common Altaic (if such a thing existed at all) — since comparison with Uralic seems to be able to show a fair number of good examples of both developments, yet strongly split according to their distribution. It does not really matter for this purpose if the comparanda are real cognates or loans … but see below for a hypothesis.
In the following, I have stuck to the clearest data, where comparison with Uralic seems, usually on semantic grounds, preferrable to or at least equally good as the proposed Altaic connections. Checking up on the non-EDAL lexicon of the languages would probably also turn up something, but I will leave that for later.
1. Turkic: *w > *b
(1.1) *bāj ‘rich, noble’ ~ Samic-Finnic *wäjä- ‘to be able, have power’, Hung. vív ‘to fight’
(not worse than ~ Mg ‘strong’, Tg ‘many’, Jp ‘to surpass’)
(1.2) *bakɨr ‘copper’ ~ PU #wäśkä ‘(reddish) metal, ? copper’ > Khanty *wăɣ ‘iron’
(rather than ~ Mg ‘patina’, Jp ‘dust’)
(1.3) *balk- ‘to shine’ ~ PU *wëlkəta ‘light, white’
(rather than ~ Mg *mel-, Tg *mial- with no **-k-; Ko *mark- may or may not belong; maybe here also Tg *beli ‘pale’, rather than ~ Mg ‘dark’)
(1.4) *bań ‘fat’ ~ PU *wajə ‘id.’
(rather than ~ Mg ‘churn’, Tg ‘storage’)
(1.5) *bek ‘firm, stable’ ~ Samic-Finnic *waka ‘id.’
(rather than ~ Mg Tg ‘big’)
(1.6) *bejŋi ‘brain’ ~ PU *wajŋə ‘breath, spirit’ > Selkup *kȫŋə ‘brain’
(rather than ~ Mg ‘forehead’)
(1.7) *bij- ‘sharp edge’ ~ Samic-Finnic *wijə- ‘to be sharp’
(rather than ~ Mg ‘to crush’, Tg ‘to mince’)
(1.8) *(b)ōl-, Mg *bol- ‘to become’, Japonic *wər- ‘to be’ ~ Uralic *(w)alə- ‘to be’ > Ob-Ugric ‘to be, to become’
(1.9) *burun ‘nose’ ~ PU *wara ‘mountain’ > Hung. orr ‘nose, †peak’
(rather than ~ Jp Ko ‘beak’)
(1.10) *būt ‘leg’ ~ Samoyedic *utå ‘hand’
(Tg *begdi may or may not belong)
(1.11) *dabul ‘wind’ ~ PU #tɜwlə ‘id.’
(rather than ~ Mg ‘typhus’, Tg ‘to be infected’)
(1.12) *debe ‘camel’ ~ Samoyedic *tëə < ? *tëwə ‘(tame) reindeer’
(rather than ~ Mg *temeɣen; long compared also with isolated Karelian tevana ‘elk cow’ (often mis-cited as Finnish))
(1.13) *sib- ‘to spin thread, pull out fibre’, Tg *sib- ‘id.’ ~ PU *siwə ‘fibre’
(rather than ~ Mg ‘to tuck up’)
I suspect most of these to be loans into Turkic from early Ugric, and in the case of *bōl-, thence into Mongolic. At least #wūta is probably better taken as a loan in the opposite direction, since this is innovative vocabulary replacing PU *kätə (and Samoyedic does not tolerate **wu-). Perhaps likewise for #dewe.
For a few IE parallels, I can moreover mention e.g. Tk *basu ‘hammer’ ~ II *wadźra- ‘hammer, mace’; Tk *ebin ‘grain’ ~ IE *yewo- ‘id.’; Tk *gēb- ‘to chew’, Mg *gebi- ‘id.’, Tg *keb- ‘to bite’ ~ IE *ǵyew- ‘to chew’. Comparison with Japonic would also immediately provide examples for *w > *b. There has been some debate on if *b or *w should be reconstructed for Proto-Japonic, but as far as I gather, *b has been assumed for ease of Altaic comparison, while most of the actual data clearly sides with *w. 
*w > *b also has good areal parallels, being found in both the north(west)ern and south(west)ern neighbors of Turkic: on one hand widely in Samoyedic, viz. in Enets, Nganasan, Kamassian and Mator (partly even in Yurats and eastern dialects of Tundra Nenets), on the other, in East Sakan (Khotanese and Tumshuqese).
There is also one notable exception where Turkic seems to have *w > ∅ instead: *öl- ‘to die’ ~ PU *widə- > Hung. *ül- > öl- ‘to kill’. This isolated example could be, however, merely an accidental similarity, esp. since the semantics are off. (‘Die’ and ‘kill’ are close enough concepts, but usually do not interchange without causative / anticausative morphology.) Contrast also ‘nose’, where we seem to have *wu > *bu in Turkic but *wu > *u > o in Hungarian.
All in all, the details may use further fine-tuning, but I think there is good evidence to assume that earlier *w develops into *b in Turkic. Contrary to what I earlier commented on this topic though, it is also easy enough to find equally good-looking cases of Turkic *b ~ Uralic *p (e.g. *bas- ‘to press’ ~ *puńćə- ‘to press, squeeze’, *beliŋ ‘panic’ ~ *pelə- ‘to fear’, *bɨč- ‘to cut’ ~ *päčkä- ‘id.’, *bulun ‘cloud’ ~ *pilw/ŋə ‘id.’), so probably this was still a merger with a pre-existing *b.
2. Mongolic: *w > ∅
Supported by less data, but even fairly tight reins on semantics still allow finding some evidence.
(2.1) *oŋgi ‘hole’, Tg *uŋgV ‘id.’ ~ PU *woŋkə ‘id.’
(rather than ~ Tk ‘to dig’)
(2.2) *ök/g- ‘to give’ ~ PU *wexə- ‘to take somewhere’ > Samoyedic *ü- ‘to drag’
(rather than ~ Tk, Tg ‘to heap up’; maybe here better Tg *bū- ‘to give’?)
(2.3) *udu- ‘to lead’ ~ PU *we/ätä- ‘to pull, lead’, PIE *wedʰ- ‘id.’
(rather than ~ Tk ‘to send’)
(2.4) *usu ‘water’ ~ PU *wetə ‘id.’
(rather than ~ Tk *sɨb)
(2.5) *üdže- ‘to see’, Tg *edže- ‘to understand’ ~ PU *weńćä- ‘to look, watch’ 
(rather than ~ Tk ‘to think, understand’; ‘understand’ is surely secondary in both etymological groups, and ‘think’ ~ ‘see’ does not match)
(2.6) *ündü-sü ‘root’ ~ PU *wanča ‘id.’
(possibly suggests that PU *č < *ts or *tU; Tg *ŋǖŋte may or may not belong)
A possible IE parallel that looks like it could have been transmitted thru Uralic: *ös ‘revenge, hate’ ~ II *dwiša- ‘hate’ (→ Permic, Finnic #wiša) (not worse than ~ Tk, Tg ‘bad, evil’). This is not attested in Ugric or Samoyedic, though, unlike all of the above examples.
The different treatment here is possibly however simply due to geography / relative chronology and not due to an actually different native development. Mongolic is a more eastern family, and may have gotten rid of *w already before contact with Uralic or some flavor of Para-Uralic — perhaps still indeed by > *b as per comparison with Turkic. So the correspondence here might indicate that in later loans, *w was substituted as zero.
I have not managed to find any reasonable-looking cases of Mongolic *b ~ Uralic *w (other than ‘to become’, see under Turkic).
The loanword layer interpretation can be also supported by how for Tungusic I cannot on a quick look-around find any clear etymologies of either type at all (i.e. where comparison with Uralic would be clearly preferrable to supposed Altaic origin). You can find some Tg cognates above under both my Turkic and Mongolic comparisons, but they might be loans. I could still add a few word-internal cases suggesting *w > *b, though: *dolba ‘night’ ~ Samoyedic *tålwə ‘dark(ness)’ (no worse than ~ Mg ‘to stay up overnight’); *nebi ‘new’ ~ PIE *new-.
 “We” being at least 90% the OP “Nortaneous” (lingblr yeli-renrong); myself probably not more than 5%, and a handful of remaining people suggesting single datapoints.
 He does not explicitly say so, and in his book leaves the Altaic column empty in the overview of Nostratic sound correspondences; but the few examples he has of a root with *w- being reflected in Altaic show zero onset.
 Well, “with rounding of the adjacent vowel”, but I would not buy any current claims about Proto-Nostratic vowel reconstruction with a nine-feet pole.
 As for Korean, the modern language has /w/, but I have the impression it mostly occurs due to vowel breaking or in loanwords from Chinese. I admit knowing very little about Middle or Old Korean though, and hence I am skipping over Korean in this post entirely.
 IMO better thus than UEW’s *wića-. Permic *dź ~ Hung. gy clearly proves *ńć, and front-harmonic cognates in these clearly prove *-ä and not *-a. Hung. front-harmonic í is also almost always from *e, not *i. Finnic can be routed as “*weŋ́śä-” > *wejśä- > *viisä-, and for Permic I suspect early *e > *i next to palatals in certain cases.